This is from the adsense policies: https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=48182 Even if your mass emails are not unsolicited you could run into problems if the visitors to your site look like that kind of traffic and/or have an unusual clicking pattern on your ads.
I wouldn't risk it. If you are doing a mass email as part of a legit marketing campaign, you should have a landing page that is free of Adsense.