diff --git a/app/email_utils.py b/app/email_utils.py
index ca5aa0414..742806de8 100644
--- a/app/email_utils.py
+++ b/app/email_utils.py
@@ -592,7 +592,7 @@ def email_can_be_used_as_mailbox(email_address: str) -> bool:
 
     from app.models import CustomDomain
 
-    if CustomDomain.get_by(domain=domain, verified=True):
+    if CustomDomain.get_by(domain=domain, is_sl_subdomain=True, verified=True):
         LOG.d("domain %s is a SimpleLogin custom domain", domain)
         return False
 
diff --git a/tests/test_email_utils.py b/tests/test_email_utils.py
index 7e133f1f0..c2cc8ab9e 100644
--- a/tests/test_email_utils.py
+++ b/tests/test_email_utils.py
@@ -90,12 +90,19 @@ def test_can_be_used_as_personal_email(flask_client):
     assert not email_can_be_used_as_mailbox("ab@sl.local")
     assert not email_can_be_used_as_mailbox("hey@d1.test")
 
-    # custom domain
+    # custom domain as SL domain
     domain = random_domain()
     user = create_new_user()
-    CustomDomain.create(user_id=user.id, domain=domain, verified=True, commit=True)
+    domain_obj = CustomDomain.create(
+        user_id=user.id, domain=domain, verified=True, is_sl_subdomain=True, flush=True
+    )
     assert not email_can_be_used_as_mailbox(f"hey@{domain}")
 
+    # custom domain is NOT SL domain
+    domain_obj.is_sl_subdomain = False
+    Session.flush()
+    assert email_can_be_used_as_mailbox(f"hey@{domain}")
+
     # disposable domain
     disposable_domain = random_domain()
     InvalidMailboxDomain.create(domain=disposable_domain, commit=True)