Override Alias.create to check in global trash first

app/errors.py

@@ -0,0 +1,3 @@
+class AliasInTrashError(Exception):
+    """raised when alias is deleted before """
+    pass

app/models.py

@@ -630,6 +630,18 @@ class Alias(db.Model, ModelMixin):
     user = db.relationship(User)
     mailbox = db.relationship("Mailbox")
 
+    @classmethod
+    def create(cls, **kw):
+        r = cls(**kw)
+
+        # make sure alias is not in global trash, i.e. DeletedAlias table
+        email = kw["email"]
+        if DeletedAlias.get_by(email=email):
+            raise AliasInTrashError
+
+        db.session.add(r)
+        return r
+
     @classmethod
     def create_new(cls, user, prefix, note=None, mailbox_id=None):
         if not prefix: