Hello,
you may be able to band-aid things by running dovecot from xinetd and using the 'instances' [0] keyword.
no, never again! I'm stigmatised by xinetd. I once tried to run the qmail-pop3-service (which is very simple and not very ressourcen-intensive compared to dovecot-imap) with xinetd and even with setting all options to unlimited, xinetd wasn't able to handle all the connections. Although it should work at any CPU load and no matter how many users connecting, it simply didn't accept any more connections while just 80-90 users were connecting concurrently. So xinetd definetely isn't a choice. (Btw.: I switched to tcpserver from djb to run qmail-pop3d which worked perfectly. But tpcserver doesn't have a connections_per_user option).
I hope Timo will add this feature in the next time, it shouldn't be that hard to track the ip-addresses that connections come from and block them if limits are reached.
Regards Marten