I used to do the same. One thing I learned- you can fight spam (and quite successfully!). But you cannot fight awfully formatted marketing emails that you or your clients require. Lots of times, these scored multiple times more than actual spam :(
Thankfully I don't need to worry about that for my use case, but you could probably solve a lot of this by whitelisting mailchimp, constant contact, and a few other providers that probably cover a large percentage of "legit" marketing mail.
