The problem wouldn't be with DA; it might be with your exim.conf file; exim is the MTA used by DA.
You'll have to come up with a specific example, with headers on the (finally received) email and log excerpts to match, before you get any specific help.
If it is your exim.conf file you might want to use my SpamBlocker exim.conf file; it has no such problems, and the SpamBlocker functionality is turned off by default.
Jeff