diff --git a/app/dashboard/views/custom_domain.py b/app/dashboard/views/custom_domain.py
index cf8a8edb..48feb806 100644
--- a/app/dashboard/views/custom_domain.py
+++ b/app/dashboard/views/custom_domain.py
@@ -43,13 +43,19 @@ def custom_domain():
                 if SLDomain.get_by(domain=new_domain):
                     flash("A custom domain cannot be a built-in domain.", "error")
                 elif CustomDomain.get_by(domain=new_domain):
-                    flash(f"{new_domain} already used", "warning")
+                    flash(f"{new_domain} already used", "error")
                 elif get_email_domain_part(current_user.email) == new_domain:
                     flash(
                         "You cannot add a domain that you are currently using for your personal email. "
                         "Please change your personal email to your real email",
                         "error",
                     )
+                elif Mailbox.filter(
+                    Mailbox.verified == True, Mailbox.email.endswith(f"@{new_domain}")
+                ).first():
+                    flash(
+                        f"{new_domain} already used in a SimpleLogin mailbox", "error"
+                    )
                 else:
                     new_custom_domain = CustomDomain.create(
                         domain=new_domain, user_id=current_user.id
diff --git a/tests/dashboard/test_custom_domain.py b/tests/dashboard/test_custom_domain.py
index d024a8cb..a143bceb 100644
--- a/tests/dashboard/test_custom_domain.py
+++ b/tests/dashboard/test_custom_domain.py
@@ -1,6 +1,7 @@
 from flask import url_for
 
 from app.db import Session
+from app.models import Mailbox
 from tests.utils import login
 
 
@@ -36,3 +37,23 @@ def test_add_domain_same_as_user_email(flask_client):
         b"You cannot add a domain that you are currently using for your personal email"
         in r.data
     )
+
+
+def test_add_domain_used_in_mailbox(flask_client):
+    """cannot add domain if it has been used in a verified mailbox"""
+    user = login(flask_client)
+    user.lifetime = True
+    Session.commit()
+
+    Mailbox.create(
+        user_id=user.id, email="mailbox@new-domain.com", verified=True, commit=True
+    )
+
+    r = flask_client.post(
+        url_for("dashboard.custom_domain"),
+        data={"form-name": "create", "domain": "new-domain.com"},
+        follow_redirects=True,
+    )
+
+    assert r.status_code == 200
+    assert b"new-domain.com already used in a SimpleLogin mailbox" in r.data