This is my second Zimbra install, the first one went flawlessly. Sending/Receiving internally and externally just fine.

This install however, is giving me an issue. I can send mail internally, and receive mail from external sites, however, I cannot send mail out. I have port 25, 465, and 587 all forwarded, and I verified that they can been seen by using canyouseeme.org as well as telnet.

I followed a combination of these guides: How to Setup Zimbra Open Source Mail Server on Ubuntu 10.04 | GeekLK :: and Zimbra 7.0.0 installation on Ubuntu 10.04 Server » Open Source Explorations for both installs. Again, let me stress that the first one went fine.

I can't seem to figure out the issue. When searching I noticed that a lot of people say that their ISP blocked port 25 so I checked on that and I've found different sites saying that my ISP does or doesn't. There isn't a clearly defined answer on that. But just in case I have SMTP submission enabled in /opt/zimbra/postfix/conf/master.cf so it should be using port 587 to send mail.

Any ideas on this would be greatly appreciated.