That is a postfix question... In it's simplest form, dbMail is just a 'mail store'... Sorry if I hurt any feelings by saying that!
I am not a debian user but have been using linux for some time... I use amavis-new with clamd plus a program called ASSP (perl based spam proxy filter placed on my firewall and not my mail server. http://assp.sourceforge.net). I get about 260 messages per day in spam, and maybe 2 get through that filter...
