What would be extremely useful is to be able to set
per-user message size limits.
It is quite useful in company environments where you might want different people or groups of people to be able to send different sized emails.
At the moment to achieve this you either need to do it per IP basis in tcp.smtp file which is quite messy or use
John Simpsons' 'qmail AUTH_CDB patch' and edit the file that is produced by vpopmail with all the addresses by hand which is equally tedious.
vpopmail doesn't play a role in sending of messages in this way. vpopmail can't monitor or control this.
This would be a patch to your qmail-smtpd server.