My syslog was filled up with errors such as this one:

    Sep 27 00:58:42 halk postfix/error[16751]: 628254C02F0: to=<mo_fax@m2osw.com>,
    relay=none, delay=1.8, delays=0.06/1.6/0/0.14, dsn=4.4.1, status=deferred
    (delivery temporarily suspended: connect to mail.m2osw.com[69.55.231.156]:25:
    Connection timed out)

I've been wordering for a while, since I can use Thunderbird with a direct connection to my server, I was not too concerned, but tonight I really needed to support those emails because I have work to do that requires it.

I verified everything, the two servers settings had not really changed much, only the IP addresses on both ends. Why would that cause a problem (assuming you also changed the IPs in my_networks and other such places in postfix?)

Well!!!!

I just switched to U-Verse and AT&T decided that it was a good idea to block port 25 so that way you don't become a spammer. Although I could understand they'd do that for individual accounts, for business accounts there is really NO REASON for such a block. So wasted 3h and I finally got the idea that what could very well be blocking port 25 was not my servers, but a server in between...

Then I found a place where they actually talk about all of that. Then they were annoying about the security and all of that, but really the guy kept calm and solved the problem after calling me back. I can now send emails again!!!

The one way that I started to be suspicious, I used the nmap command like this to test the SMTP port:

# Internal -> Internal
nmap -p 25 162.226.130.121
PORT   STATE SERVICE
25/tcp open  smtp

# External -> External
nmap -p 25 69.55.231.156
PORT   STATE SERVICE
25/tcp open  smtp

# External -> Internal
nmap -p 25 162.226.130.121
PORT   STATE SERVICE
25/tcp open  smtp

# Internal -> External
nmap -p 25 69.55.231.156
PORT   STATE    SERVICE
25/tcp filtered smtp

As you can see, when I was trying to connect from my computer (Internal) to my server (External) it was telling me that the port was filtered. This more or less means they DROP the TCP/IP packets. This generates a Time Out as I could see in the log. Since I have no direct control over that firewall, I had no idea that it was even there. But after they opened it, I got some 1,200 emails popping in Thunderbird.