diff --git a/app/api/views/mailbox.py b/app/api/views/mailbox.py
index e3ca02f2..866f0685 100644
--- a/app/api/views/mailbox.py
+++ b/app/api/views/mailbox.py
@@ -112,6 +112,13 @@ def update_mailbox(mailbox_id):
     if "default" in data:
         is_default = data.get("default")
         if is_default:
+            if not mailbox.verified:
+                return (
+                    jsonify(
+                        error="Unverified mailbox cannot be used as default mailbox"
+                    ),
+                    400,
+                )
             user.default_mailbox_id = mailbox.id
             changed = True
 
diff --git a/tests/api/test_mailbox.py b/tests/api/test_mailbox.py
index cccfecd2..8b502076 100644
--- a/tests/api/test_mailbox.py
+++ b/tests/api/test_mailbox.py
@@ -73,31 +73,33 @@ def test_delete_default_mailbox(flask_client):
 
 
 def test_set_mailbox_as_default(flask_client):
-    user = User.create(
-        email="a@b.c", password="password", name="Test User", activated=True
+    user = login(flask_client)
+
+    mb = Mailbox.create(
+        user_id=user.id, email="mb@gmail.com", verified=True, commit=True
     )
-    db.session.commit()
-
-    # create api_key
-    api_key = ApiKey.create(user.id, "for test")
-    db.session.commit()
-
-    # create a mailbox
-    mb = Mailbox.create(user_id=user.id, email="mb@gmail.com")
-    db.session.commit()
     assert user.default_mailbox_id != mb.id
 
     r = flask_client.put(
         f"/api/mailboxes/{mb.id}",
-        headers={"Authentication": api_key.code},
         json={"default": True},
     )
 
     assert r.status_code == 200
-
-    mb = Mailbox.get(mb.id)
     assert user.default_mailbox_id == mb.id
 
+    # <<< Cannot set an unverified mailbox as default >>>
+    mb.verified = False
+    db.session.commit()
+
+    r = flask_client.put(
+        f"/api/mailboxes/{mb.id}",
+        json={"default": True},
+    )
+
+    assert r.status_code == 400
+    assert r.json == {"error": "Unverified mailbox cannot be used as default mailbox"}
+
 
 def test_update_mailbox_email(flask_client):
     user = User.create(