A bit off-topic, but Edvard, what are you doing exchanging large graphics files through email? Email is about the worst Internet file transfer system that exists. There are a lot of easy (and secure, if necessary) alternatives. Granted email is a tad easier than even the easiest of the alternatives, generally speaking, but it also bloats file size about 30% and the simple fact is that it was never meant to be used as a file transfer system. That means that every tool you're using with your email or that ever looks at your email, from the client itself to your antivirus and antispam software, has to deal with unusually large, burdensome emails. Sure they shouldn't bog down when dealing with that stuff, but you're also using the system in a way it wasn't intended for so it's not really a surprise to me that performance isn't optimal.
There are at least 3 major counts against email as a file transfer system:
Larger file size - about 30% bigger on average
Inability to resume downloads in most cases
Email system not designed for attachments, negatively affects any program that interacts with it
Given that I think it makes sense to look for alternatives, as a separate issue from your spam filtering of course. You might find it helps with that too though.
I don't know what your specific needs are, whether there is any possibility for a central FTP repository or anything, but I definitely suggest looking into alternatives for file transfer. Something like HFS
may work very well, depending on your needs (I love it personally, and I do use it in similar situations).
Sorry I've nothing to contribute on the original topic though.