Before I sent out my message, I tested with my gmail account, and it went through just fine (and on another account as well). However, when I finally sent the campaign message to my whole list, apparently my messages were flagged as phishing on the gmail servers. Unfortunately, this was about a 1/3rd of my customers.
Any tips on how to avoid this in the future?
Typically phishing warnings are from having a mismatch between a URL and the actual link text - see http://www.campaignmonitor.com/blog/post/2000/how-to-avoid-looking-like-a-ph/ for more.