diff --git a/application/Espo/Classes/MassAction/User/MassDelete.php b/application/Espo/Classes/MassAction/User/MassDelete.php
index cd470fc401..c6ca174c39 100644
--- a/application/Espo/Classes/MassAction/User/MassDelete.php
+++ b/application/Espo/Classes/MassAction/User/MassDelete.php
@@ -29,48 +29,36 @@
 
 namespace Espo\Classes\MassAction\User;
 
-use Espo\Core\{
-    ApplicationUser,
-    MassAction\Actions\MassDelete as MassDeleteOriginal,
-    MassAction\QueryBuilder,
-    MassAction\Params,
-    MassAction\Result,
-    MassAction\Data,
-    MassAction\MassAction,
-    Acl,
-    ORM\EntityManager,
-    Exceptions\Forbidden,
-};
+use Espo\Core\Acl;
+use Espo\Core\ApplicationUser;
+use Espo\Core\Exceptions\BadRequest;
+use Espo\Core\Exceptions\Forbidden;
+use Espo\Core\MassAction\Actions\MassDelete as MassDeleteOriginal;
+use Espo\Core\MassAction\Data;
+use Espo\Core\MassAction\MassAction;
+use Espo\Core\MassAction\Params;
+use Espo\Core\MassAction\QueryBuilder;
+use Espo\Core\MassAction\Result;
+use Espo\Core\ORM\EntityManager;
 
-use Espo\{
-    Entities\User,
-    ORM\Entity,
-};
+use Espo\Entities\User;
 
+/**
+ * Extended to forbid removal of own and system users.
+ */
 class MassDelete implements MassAction
 {
-    private MassDeleteOriginal $massDeleteOriginal;
-    private QueryBuilder $queryBuilder;
-    private EntityManager $entityManager;
-    private Acl $acl;
-    private User $user;
-
     public function __construct(
-        MassDeleteOriginal $massDeleteOriginal,
-        QueryBuilder $queryBuilder,
-        EntityManager $entityManager,
-        Acl $acl,
-        User $user
-    ) {
-        $this->massDeleteOriginal = $massDeleteOriginal;
-        $this->queryBuilder = $queryBuilder;
-        $this->entityManager = $entityManager;
-        $this->acl = $acl;
-        $this->user = $user;
-    }
+        private MassDeleteOriginal $massDeleteOriginal,
+        private QueryBuilder $queryBuilder,
+        private EntityManager $entityManager,
+        private Acl $acl,
+        private User $user
+    ) {}
 
     /**
      * @throws Forbidden
+     * @throws BadRequest
      */
     public function process(Params $params, Data $data): Result
     {
@@ -93,7 +81,7 @@ class MassDelete implements MassAction
             ->getRDBRepository(User::ENTITY_TYPE)
             ->clone($query)
             ->sth()
-            ->select(['id'])
+            ->select(['id', 'userName'])
             ->find();
 
         foreach ($collection as $entity) {
@@ -106,9 +94,9 @@ class MassDelete implements MassAction
     /**
      * @throws Forbidden
      */
-    protected function checkEntity(Entity $entity): void
+    private function checkEntity(User $entity): void
     {
-        if ($entity->getId() === ApplicationUser::SYSTEM_USER_ID) {
+        if ($entity->getUserName() === ApplicationUser::SYSTEM_USER_NAME) {
             throw new Forbidden("Can't delete 'system' user.");
         }
 
diff --git a/application/Espo/Classes/MassAction/User/MassUpdate.php b/application/Espo/Classes/MassAction/User/MassUpdate.php
index 8af89198e3..2a3788b778 100644
--- a/application/Espo/Classes/MassAction/User/MassUpdate.php
+++ b/application/Espo/Classes/MassAction/User/MassUpdate.php
@@ -30,6 +30,7 @@
 namespace Espo\Classes\MassAction\User;
 
 use Espo\Core\ApplicationUser;
+use Espo\Core\Exceptions\BadRequest;
 use Espo\Core\MassAction\Actions\MassUpdate as MassUpdateOriginal;
 use Espo\Core\MassAction\QueryBuilder;
 use Espo\Core\MassAction\Params;
@@ -44,7 +45,6 @@ use Espo\Core\Acl\Table;
 use Espo\Core\Exceptions\Forbidden;
 
 use Espo\Entities\User;
-use Espo\ORM\Entity;
 use Espo\ORM\EntityManager;
 
 use Espo\Tools\MassUpdate\Data as MassUpdateData;
@@ -75,6 +75,7 @@ class MassUpdate implements MassAction
 
     /**
      * @throws Forbidden
+     * @throws BadRequest
      */
     public function process(Params $params, Data $data): Result
     {
@@ -102,7 +103,7 @@ class MassUpdate implements MassAction
             ->getRDBRepository(User::ENTITY_TYPE)
             ->clone($query)
             ->sth()
-            ->select(['id'])
+            ->select(['id', 'userName'])
             ->find();
 
         foreach ($collection as $entity) {
@@ -131,9 +132,9 @@ class MassUpdate implements MassAction
     /**
      * @throws Forbidden
      */
-    private function checkEntity(Entity $entity, MassUpdateData $data): void
+    private function checkEntity(User $entity, MassUpdateData $data): void
     {
-        if ($entity->getId() === ApplicationUser::SYSTEM_USER_ID) {
+        if ($entity->getUserName() === ApplicationUser::SYSTEM_USER_NAME) {
             throw new Forbidden("Can't update 'system' user.");
         }
 
diff --git a/application/Espo/Core/ApplicationUser.php b/application/Espo/Core/ApplicationUser.php
index 4421079872..833e424c63 100644
--- a/application/Espo/Core/ApplicationUser.php
+++ b/application/Espo/Core/ApplicationUser.php
@@ -40,6 +40,7 @@ use RuntimeException;
 class ApplicationUser
 {
     public const SYSTEM_USER_ID = 'system';
+    public const SYSTEM_USER_NAME = 'system';
 
     public function __construct(
         private Container $container,
@@ -63,7 +64,7 @@ class ApplicationUser
                 'lastName',
                 'deleted',
             ])
-            ->where(['id' => self::SYSTEM_USER_ID])
+            ->where(['userName' => self::SYSTEM_USER_NAME])
             ->findOne();
 
         if (!$user) {
diff --git a/application/Espo/Core/Rebuild/Actions/AddSystemUser.php b/application/Espo/Core/Rebuild/Actions/AddSystemUser.php
index 20ef813d70..2a7972d911 100644
--- a/application/Espo/Core/Rebuild/Actions/AddSystemUser.php
+++ b/application/Espo/Core/Rebuild/Actions/AddSystemUser.php
@@ -44,22 +44,28 @@ class AddSystemUser implements RebuildAction
 
     public function process(): void
     {
-        $userId = ApplicationUser::SYSTEM_USER_ID;
-
         $repository = $this->entityManager->getRDBRepositoryByClass(User::class);
 
-        $user = $repository->getById($userId);
+        $user = $repository
+            ->where(['userName' => ApplicationUser::SYSTEM_USER_NAME])
+            ->findOne();
 
         if ($user) {
             return;
         }
 
+        // @todo If a user with the 'system' ID already exists, delete it from DB.
+
         /** @var array<string, mixed> $attributes */
         $attributes = $this->config->get('systemUserAttributes');
 
         $user = $repository->getNew();
+
+        $user->set('id', ApplicationUser::SYSTEM_USER_ID);
+        $user->set('userName', ApplicationUser::SYSTEM_USER_NAME);
+        $user->set('type', User::TYPE_SYSTEM);
         $user->set($attributes);
-        $user->set('id', $userId);
+
         $repository->save($user);
     }
 }
diff --git a/application/Espo/Entities/Email.php b/application/Espo/Entities/Email.php
index 2f1badbf17..d39aec198e 100644
--- a/application/Espo/Entities/Email.php
+++ b/application/Espo/Entities/Email.php
@@ -169,8 +169,15 @@ class Email extends Entity
         $this->setInContainer('isUsers', false);
     }
 
+    /**
+     * @deprecated As of v7.4. As the system user ID may be not constant in the future.
+     */
     public function isManuallyArchived(): bool
     {
+        if ($this->getStatus() !== self::STATUS_ARCHIVED) {
+            return false;
+        }
+
         return $this->getStatus() === self::STATUS_ARCHIVED &&
             $this->get('createdById') !== ApplicationUser::SYSTEM_USER_ID;
     }
diff --git a/application/Espo/EntryPoints/Avatar.php b/application/Espo/EntryPoints/Avatar.php
index 616e0d4d76..28fe9944b8 100644
--- a/application/Espo/EntryPoints/Avatar.php
+++ b/application/Espo/EntryPoints/Avatar.php
@@ -100,6 +100,7 @@ class Avatar extends Image
             throw new BadRequest();
         }
 
+        /** @var ?User $user */
         $user = $this->entityManager->getEntityById(User::ENTITY_TYPE, $userId);
 
         if (!$user) {
@@ -112,6 +113,8 @@ class Avatar extends Image
 
         if ($id) {
             $this->show($response, $id, $size, true);
+
+            return;
         }
 
         $identicon = new Identicon();
@@ -136,7 +139,7 @@ class Avatar extends Image
 
         $color = $this->getColor($userId);
 
-        if ($hash === ApplicationUser::SYSTEM_USER_ID) {
+        if ($user->getUserName() === ApplicationUser::SYSTEM_USER_NAME) {
             $color = $this->metadata->get(['app', 'avatars', 'systemColor']) ?? $this->systemColor;
         }
 
diff --git a/application/Espo/Resources/defaults/systemConfig.php b/application/Espo/Resources/defaults/systemConfig.php
index 95320c2131..6a563aff45 100644
--- a/application/Espo/Resources/defaults/systemConfig.php
+++ b/application/Espo/Resources/defaults/systemConfig.php
@@ -42,10 +42,7 @@ return [
         'delete' => 'delete',
     ],
     'systemUserAttributes' => [
-        'userName' => 'system',
-        'firstName' => '',
         'lastName' => 'System',
-        'type' => 'system',
     ],
     'systemItems' => [
         'systemItems',
diff --git a/application/Espo/Services/Email.php b/application/Espo/Services/Email.php
index 28a0a6d2f4..a143dc9bc8 100644
--- a/application/Espo/Services/Email.php
+++ b/application/Espo/Services/Email.php
@@ -29,6 +29,7 @@
 
 namespace Espo\Services;
 
+use Espo\Core\ApplicationUser;
 use Espo\Tools\Email\SendService;
 use Espo\ORM\Entity;
 use Espo\Entities\User;
@@ -211,12 +212,11 @@ class Email extends Record
             $skipFilter = true;
         }
 
-        if ($entity->isManuallyArchived()) {
+        if ($this->isEmailManuallyArchived($entity)) {
             $skipFilter = true;
-        } else {
-            if ($entity->isAttributeChanged('dateSent')) {
-                $entity->set('dateSent', $entity->getFetched('dateSent'));
-            }
+        }
+        else if ($entity->isAttributeChanged('dateSent')) {
+            $entity->set('dateSent', $entity->getFetched('dateSent'));
         }
 
         if ($entity->getStatus() === EmailEntity::STATUS_DRAFT) {
@@ -248,6 +248,30 @@ class Email extends Record
         }
     }
 
+    private function isEmailManuallyArchived(EmailEntity $email): bool
+    {
+        if ($email->getStatus() !== EmailEntity::STATUS_ARCHIVED) {
+            return false;
+        }
+
+        $userId = $email->getCreatedBy()?->getId();
+
+        if (!$userId) {
+            return false;
+        }
+
+        /** @var ?User $user */
+        $user = $this->entityManager
+            ->getRDBRepositoryByClass(User::class)
+            ->getById($userId);
+
+        if (!$user) {
+            return true;
+        }
+
+        return $user->getUserName() !== ApplicationUser::SYSTEM_USER_NAME;
+    }
+
     private function clearEntityForUpdate(EmailEntity $email): void
     {
         $fieldDefsList = $this->entityManager
diff --git a/application/Espo/Services/User.php b/application/Espo/Services/User.php
index 468ff6fe64..5e7ba03e0a 100644
--- a/application/Espo/Services/User.php
+++ b/application/Espo/Services/User.php
@@ -89,18 +89,18 @@ class User extends Record implements
      */
     public function getEntity(string $id): ?Entity
     {
-        if ($id === ApplicationUser::SYSTEM_USER_ID) {
-            throw new Forbidden();
-        }
-
         /** @var ?UserEntity $entity */
         $entity = parent::getEntity($id);
 
-        if ($entity && $entity->isSuperAdmin() && !$this->user->isSuperAdmin()) {
+        if (!$entity) {
+            return null;
+        }
+
+        if ($entity->isSuperAdmin() && !$this->user->isSuperAdmin()) {
             throw new Forbidden();
         }
 
-        if ($entity && $entity->isSystem()) {
+        if ($entity->isSystem()) {
             throw new Forbidden();
         }
 
@@ -176,10 +176,6 @@ class User extends Record implements
 
     public function update(string $id, stdClass $data, UpdateParams $params): Entity
     {
-        if ($id === ApplicationUser::SYSTEM_USER_ID) {
-            throw new Forbidden();
-        }
-
         $newPassword = null;
 
         if (property_exists($data, 'password')) {
@@ -192,7 +188,7 @@ class User extends Record implements
             $data->password = $this->hashPassword($data->password);
         }
 
-        if ($id == $this->user->getId()) {
+        if ($id === $this->user->getId()) {
             unset($data->isActive);
             unset($data->isPortalUser);
             unset($data->type);
@@ -207,7 +203,7 @@ class User extends Record implements
                     $this->sendPassword($user, $newPassword);
                 }
             }
-            catch (Exception $e) {}
+            catch (Exception) {}
         }
 
         return $user;
@@ -281,43 +277,41 @@ class User extends Record implements
     }
 
     /**
+     * @param UserEntity $entity
      * @throws Forbidden
      */
     protected function beforeCreateEntity(Entity $entity, $data)
     {
-        /** @var UserEntity $entity */
+        $userLimit = $this->config->get('userLimit');
 
         if (
-            $this->config->get('userLimit') &&
+            $userLimit &&
             !$this->user->isSuperAdmin() &&
             !$entity->isPortal() && !$entity->isApi()
         ) {
             $userCount = $this->getInternalUserCount();
 
-            if ($userCount >= $this->config->get('userLimit')) {
-                throw new Forbidden(
-                    'User limit '.$this->config->get('userLimit').' is reached.'
-                );
+            if ($userCount >= $userLimit) {
+                throw new Forbidden("User limit {$userLimit} is reached.");
             }
         }
+
+        $portalUserLimit = $this->config->get('portalUserLimit');
+
         if (
-            $this->config->get('portalUserLimit') &&
+            $portalUserLimit &&
             !$this->user->isSuperAdmin() &&
             $entity->isPortal()
         ) {
             $portalUserCount = $this->getPortalUserCount();
 
-            if ($portalUserCount >= $this->config->get('portalUserLimit')) {
-                throw new Forbidden(
-                    'Portal user limit ' . $this->config->get('portalUserLimit').' is reached.'
-                );
+            if ($portalUserCount >= $portalUserLimit) {
+                throw new Forbidden("Portal user limit {$portalUserLimit} is reached.");
             }
         }
 
         if ($entity->isApi()) {
-            $apiKey = Util::generateApiKey();
-
-            $entity->set('apiKey', $apiKey);
+            $entity->set('apiKey', Util::generateApiKey());
 
             if ($entity->getAuthMethod() === Hmac::NAME) {
                 $secretKey = Util::generateSecretKey();
@@ -326,25 +320,27 @@ class User extends Record implements
             }
         }
 
-        if (!$entity->isSuperAdmin()) {
-            if (
-                $entity->getType() &&
-                !in_array($entity->getType(), $this->allowedUserTypeList)
-            ) {
-                throw new Forbidden();
-            }
+        if (
+            !$entity->isSuperAdmin() &&
+            $entity->getType() &&
+            !in_array($entity->getType(), $this->allowedUserTypeList)
+        ) {
+            throw new Forbidden();
         }
     }
 
     /**
+     * @param UserEntity $entity
      * @throws Forbidden
      */
     protected function beforeUpdateEntity(Entity $entity, $data)
     {
-        /** @var UserEntity $entity */
+        $userLimit = $this->config->get('userLimit');
 
-        if ($this->config->get('userLimit') && !$this->user->isSuperAdmin()) {
-            if (
+        if (
+            $userLimit &&
+            !$this->user->isSuperAdmin() &&
+            (
                 (
                     $entity->isActive() &&
                     $entity->isAttributeChanged('isActive') &&
@@ -364,17 +360,21 @@ class User extends Record implements
                         $entity->getFetched('type') == UserEntity::TYPE_API
                     )
                 )
-            ) {
-                $userCount = $this->getInternalUserCount();
+            )
+        ) {
+            $userCount = $this->getInternalUserCount();
 
-                if ($userCount >= $this->config->get('userLimit')) {
-                    throw new Forbidden('User limit '.$this->config->get('userLimit').' is reached.');
-                }
+            if ($userCount >= $userLimit) {
+                throw new Forbidden("User limit {$userLimit} is reached.");
             }
         }
 
-        if ($this->config->get('portalUserLimit') && !$this->user->isSuperAdmin()) {
-            if (
+        $portalUserLimit = $this->config->get('portalUserLimit');
+
+        if (
+            $portalUserLimit &&
+            !$this->user->isSuperAdmin() &&
+            (
                 (
                     $entity->isActive() &&
                     $entity->isAttributeChanged('isActive') &&
@@ -384,47 +384,39 @@ class User extends Record implements
                     $entity->isPortal() &&
                     $entity->isAttributeChanged('type')
                 )
-            ) {
-                $portalUserCount = $this->getPortalUserCount();
+            )
+        ) {
+            $portalUserCount = $this->getPortalUserCount();
 
-                if ($portalUserCount >= $this->config->get('portalUserLimit')) {
-                    throw new Forbidden(
-                        'Portal user limit '. $this->config->get('portalUserLimit').' is reached.'
-                    );
-                }
+            if ($portalUserCount >= $portalUserLimit) {
+                throw new Forbidden("Portal user limit {$portalUserLimit} is reached.");
             }
         }
 
-        if ($entity->isApi()) {
-            if (
-                $entity->isAttributeChanged('authMethod') &&
-                $entity->getAuthMethod() === Hmac::NAME
-            ) {
-                $secretKey = Util::generateSecretKey();
+        if (
+            $entity->isApi() &&
+            $entity->isAttributeChanged('authMethod') &&
+            $entity->getAuthMethod() === Hmac::NAME
+        ) {
+            $secretKey = Util::generateSecretKey();
 
-                $entity->set('secretKey', $secretKey);
-            }
+            $entity->set('secretKey', $secretKey);
         }
 
-        if (!$entity->isSuperAdmin()) {
-            if (
-                $entity->isAttributeChanged('type') &&
-                $entity->getType() &&
-                !in_array($entity->getType(), $this->allowedUserTypeList)
-            ) {
-                throw new Forbidden();
-            }
+        if (
+            !$entity->isSuperAdmin() &&
+            $entity->isAttributeChanged('type') &&
+            $entity->getType() &&
+            !in_array($entity->getType(), $this->allowedUserTypeList)
+        ) {
+            throw new Forbidden("Can't change type.");
         }
     }
 
     public function delete(string $id, DeleteParams $params): void
     {
-        if ($id === ApplicationUser::SYSTEM_USER_ID) {
-            throw new Forbidden();
-        }
-
-        if ($id == $this->user->getId()) {
-            throw new Forbidden();
+        if ($id === $this->user->getId()) {
+            throw new Forbidden("Can't delete own user.");
         }
 
         parent::delete($id, $params);
diff --git a/application/Espo/Tools/MassUpdate/MassUpdate.php b/application/Espo/Tools/MassUpdate/MassUpdate.php
index 6ba0c88ec0..cf6a3ded6a 100644
--- a/application/Espo/Tools/MassUpdate/MassUpdate.php
+++ b/application/Espo/Tools/MassUpdate/MassUpdate.php
@@ -30,6 +30,8 @@
 namespace Espo\Tools\MassUpdate;
 
 use Espo\Core\ApplicationUser;
+use Espo\Core\Exceptions\BadRequest;
+use Espo\Core\Exceptions\Forbidden;
 use Espo\Core\Exceptions\NotFound;
 use Espo\Core\MassAction\Params;
 use Espo\Core\MassAction\Result;
@@ -45,29 +47,31 @@ use RuntimeException;
  */
 class MassUpdate
 {
-    private MassActionFactory $massActionFactory;
-    private EntityManager $entityManager;
-
     private const ACTION = 'massUpdate';
-    private const DEFAULT_USER_ID = ApplicationUser::SYSTEM_USER_ID;
 
-    public function __construct(MassActionFactory $massActionFactory, EntityManager $entityManager)
-    {
-        $this->massActionFactory = $massActionFactory;
-        $this->entityManager = $entityManager;
-    }
+    public function __construct(
+        private MassActionFactory $massActionFactory,
+        private EntityManager $entityManager
+    ) {}
 
     /**
+     * Process.
+     *
      * @param ?User $user Under what user to perform mass-update. If not specified, the system user will be used.
      * Access control is applied for the user.
      * @throws NotFound
+     * @throws Forbidden
+     * @throws BadRequest
      */
     public function process(Params $params, Data $data, ?User $user = null): Result
     {
         $entityType = $params->getEntityType();
 
         if (!$user) {
-            $user = $this->entityManager->getEntityById(User::ENTITY_TYPE, self::DEFAULT_USER_ID);
+            $user = $this->entityManager
+                ->getRDBRepositoryByClass(User::class)
+                ->where(['userName' => ApplicationUser::SYSTEM_USER_NAME])
+                ->findOne();
         }
 
         if (!$user) {