diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5cd0c2d..61b180a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ and this project adheres to
 
 ### Changed
 
+- 🚸(email) we should ignore case when looking for existing emails #1056
 - 🏗️(core) migrate from pip to uv
 - ✨(front) add show invitations mails domains access #1040
 
diff --git a/src/backend/core/models.py b/src/backend/core/models.py
index cf8ccf1..33a25af 100644
--- a/src/backend/core/models.py
+++ b/src/backend/core/models.py
@@ -998,7 +998,7 @@ class BaseInvitation(BaseModel):
         super().clean()
 
         # Check if a user already exists for the provided email
-        if User.objects.filter(email=self.email).exists():
+        if User.objects.filter(email__iexact=self.email).exists():
             raise EmailAlreadyKnownException
 
     @property
diff --git a/src/backend/core/tests/test_models_invitations.py b/src/backend/core/tests/test_models_invitations.py
index 936f3c8..73fe414 100644
--- a/src/backend/core/tests/test_models_invitations.py
+++ b/src/backend/core/tests/test_models_invitations.py
@@ -17,6 +17,8 @@ from freezegun import freeze_time
 
 from core import factories, models
 
+from mailbox_manager.exceptions import EmailAlreadyKnownException
+
 pytestmark = pytest.mark.django_db
 
 
@@ -48,6 +50,17 @@ def test_models_invitations_team_required():
         factories.InvitationFactory(team=None)
 
 
+def test_models_invitations_email_case_insensitive_duplicate_check():
+    """The email validation should be case-insensitive when checking for existing users."""
+    # Create a user with a lowercase email
+    factories.UserFactory(email="john.doe@example.com")
+
+    # Try to create an invitation with different case
+    # This should raise the same exception as if the email was exactly the same
+    with pytest.raises(EmailAlreadyKnownException):
+        factories.InvitationFactory(email="John.Doe@Example.COM")
+
+
 def test_models_invitations_team_should_be_team_instance():
     """The "team" field should be a team instance."""
     with pytest.raises(ValueError, match='Invitation.team" must be a "Team" instance'):
diff --git a/src/backend/mailbox_manager/api/client/viewsets.py b/src/backend/mailbox_manager/api/client/viewsets.py
index be30d71..f032dc5 100644
--- a/src/backend/mailbox_manager/api/client/viewsets.py
+++ b/src/backend/mailbox_manager/api/client/viewsets.py
@@ -419,7 +419,7 @@ class MailDomainInvitationViewset(
         try:
             return super().create(request, *args, **kwargs)
         except EmailAlreadyKnownException as exc:
-            user = models.User.objects.get(email=email)
+            user = models.User.objects.get(email__iexact=email)
 
             models.MailDomainAccess.objects.create(
                 user=user,
diff --git a/src/backend/mailbox_manager/tests/api/invitations/test_api_domain_invitations_create.py b/src/backend/mailbox_manager/tests/api/invitations/test_api_domain_invitations_create.py
index 9799599..8514d17 100644
--- a/src/backend/mailbox_manager/tests/api/invitations/test_api_domain_invitations_create.py
+++ b/src/backend/mailbox_manager/tests/api/invitations/test_api_domain_invitations_create.py
@@ -158,3 +158,40 @@ def test_api_domain_invitations__inviting_known_email_should_create_access():
 
     assert not models.MailDomainInvitation.objects.exists()
     assert models.MailDomainAccess.objects.filter(user=existing_user).exists()
+
+
+def test_api_domain_invitations__inviting_known_email_case_insensitive():
+    """Email matching should be case-insensitive when creating access for existing users."""
+    # Create a user with lowercase email
+    existing_user = core_factories.UserFactory(email="john.doe@example.com")
+    access = factories.MailDomainAccessFactory(role=enums.MailDomainRoleChoices.OWNER)
+
+    client = APIClient()
+    client.force_login(access.user)
+
+    # Try to invite with same email different case - should create access for existing user
+    response = client.post(
+        f"/api/v1.0/mail-domains/{access.domain.slug}/invitations/",
+        {
+            "email": "John.Doe@Example.COM",
+            "role": "owner",
+        },
+        format="json",
+    )
+    assert response.status_code == status.HTTP_201_CREATED
+    assert (
+        response.json()["detail"]
+        == "Email already known. Invitation not sent but access created instead."
+    )
+
+    # No invitation should be created
+    assert not models.MailDomainInvitation.objects.exists()
+
+    # Access should be created for the existing user (not a new user)
+    assert models.MailDomainAccess.objects.filter(user=existing_user).exists()
+    assert models.MailDomainAccess.objects.filter(
+        user=existing_user, domain=access.domain
+    ).exists()
+
+    # Ensure only one user exists (no duplicate user created)
+    assert models.User.objects.filter(email__iexact="john.doe@example.com").count() == 1