Excellent, thanks. I should try this out. The project has moved to Github...
One thing I am concerned about, in general, is ram usage and both amavisd and spamassassin use a lot of ram and even though the virus checking is useful for windows based clients I find spamassassin sucks at filtering out spam with it's "Normal" setting (I'm not sure what the other ISPConfig settings actually do). No doubt a lot of RTFM would sort that out and adding even more modules like Dspam would help. However I find an untrained Dspam setup will outperform spamassassin and when trained will get over 99% of spam and can adapt to each user mail patterns.
~ ps auxf | sort -nr -k 4 | head -n 4
clamav 21069 0.0 4.9 468892 199520 ? Ssl Apr11 3:38 /usr/sbin/clamd
amavis 14699 0.1 2.5 304396 103636 ? S 00:15 0:01 \_ /usr/sbin/amavisd-new (ch1-avail)
amavis 13259 0.0 2.4 301544 100488 ? S Apr17 0:01 \_ /usr/sbin/amavisd-new (ch3-avail)
amavis 20290 0.0 2.3 221664 96892 ? Ss Apr11 0:04 /usr/sbin/amavisd-new (master)
The above is taking up 500Mb of real ram on my main server compared to Dspam being called from maildrop as needed (no daemon) taking up zero ram except when actually in use. Also, regarding courier-mta, if using courier-imap and it's authdaemon then courier-mta uses the same authdeamon as courier-imap and the couriersmtpd processes use ~1Mb ram compared to posfix at ~5Mb per instance. Same with courier-imap at ~1.5Mb compared to dovecot at around 3Mb per process.
So, in summary, courier-mta/imap plus Dspam from maildrop can offer significant ram savings for either small servers or run higher levels of service on large servers. Better "bang for buck" IMHO.