Welcome, confirmation, and notification emails all use "internal PHP mail()" even with SMTP enabled. This is to prevent too many connections to the SMTP server. Try changing your mailing method to "internal PHP mail()" instead of SMTP and see if you can get a message through to a test list. If you can't, you might want to talk to your host about the availability of this PHP function.
If you continue to have troubles I can run some tests on your server. Rarely, certain "send from" and "bounce to" addresses can cause problems with mailing.
Regards