I have a web based application that has a mass e-mailing component. Since the program went live, the mailing module has been plagued with message delivery issues. Users with AOL addresses don't get the messages at all and free e-mail services (like Yahoo and Hotmail) constantly treat the messages as SPAM. I have tried everything I can think of to combat the problem with no success.

I'm finally to the point where I'm considering subscribing to a SMTP service provider and sending the messages out using their service to see if that helps. Does anyone know of any good reputable providers? I'm looking for one that I can use with a program and that doesn't handle authentication by sender (because the user is going to use their own address and I won't know what it is).

I have already come across SMTP.com (which hasn't gotten back to my information request yet) and authSMTP (http://www.authsmtp.com/) who handles authentication via sender address which won't work.

Any recommendations would be greatly appreciated!