By default, this cannot be done. If you know how to write Perl, it shouldn't be too hard to do. To start you off, you would need to add two columns to the user table: one for the number of emails they can send, and the number of emails sent. All the code used to send is in Compose.pm - you would put your bit of code to check their limit inside _send(). Lastly you would need to reset the user's sent mail count once a day.

Adrian