Hi,
I see http://johannes.sipsolutions.net/Projects/dovecot-antispam and http://blog.cynapses.org/2007/09/13/dovecot-dspam-plugin/ solutions. Second git repo is not answer, first one is cloned, but I see some drawbacks with it.
Dspam backend is more native solution but libdspam using instead of dspam external binary looks like preffered way to integrate dspam. Another problem is waiting for dspam child process exit, so IMAP session is blocked. Why not to fork and forget dspam child process?
Email sender backend is non-blocking, but using MTA is great overhead. Calling external program with some args and mail body via pipe can be more suitable way, so external program may be dspam binary directly or shell/perl/python filter to any antispam engine via sendmail or something else.
Has anybody plans to implement this features or it will be better to try it myself?