limiting mailbox size

What I want to do is this:

If a mail message is to be delivered to a file and the file size
plus the message size is over a configurable limit, defer
delivery. If it's deferred for too long, return it to the sender
with an appropriate message.

Why do we need this? Well, when a user's mailbox gets to 7M or
so, POP3 clients start timing out and we start getting support
calls. :-) The idea is leave the messages in the mail queue until
the user collects his GIFs or whatever and then deliver new mail.
(Yeah, to make this work, individual mail messages must be
smaller than the limit....I was thinking 3M mail messages and 5M
mailbox size.)

Any easy way to do this, or does it require programming?