[andrewjshults]

Do you have open/click tracking on the emails? If not you should either roll your own or use someone like sendgrid who can automate the process for you (depending on how many emails you send out, even the free tier of sendgrid might be enough). Open tracking isn't perfect but it'll help you narrow down where in the process the drop off happens (sendgrid also does spam report tracking and bounces). They also put a lot of effort into making sure that your email get through (not affiliated, just a happy customer).

[aderaynal]

Thanks for the suggestions. The free tier would not be enough for me, but I don't mind spending $9.95 per month.

Does anyone have experience integrating SendGrid with a Google AppEngine Java application ?