postfix policy daemon limiting the number of mails a user can send

 policyd-rate-limit is a simple postfix policy daemon written in python3
 allowing to limit the number of mails a user can send over time.
 Users are identified either via their sasl usernames or their ip addresses.
 Limitation rules are a list of couples (number of mails, number of seconds).
 If a user has sent more than number of mails in number of seconds,
 a configurable error is returned to the user.