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I had postfix up and running, and I think it was complete, although there was still a little testing to do. But at some point, it stopped accepting connections from my home computer. I tried to telnet to the mail server on port 25 and I get a timeout. But if I telnet into it from an iphone app from the cell network, i get the 220 message that it's connected. This happened once, and a few days later i was able to connect again for a few hours, but then it stopped responding again. Meanwhile, I can still connect from other locations.

I tried to watch the connection with tcpdump, and i can see the connection coming in from the cell phone but not from the home network. So by my first guess it sounds like the firewall is blocking my IP. but I can still access https from my home network, so it's not the firewall unless it's only blocking a certain port from a certain IP. I have asked the data center to check the firewall several times and they assure me that it's not what's blocking me.

Is there are setting in postfix that would deny a specific IP for a while? maybe because of too many failed auth attempts? I still don't think that makes sense because if that were the case I should see something in tcpdump, but i'm out of ideas.

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Your home ISP is likely blocking 25/tcp. This is quite common. –  EEAA Dec 22 '12 at 3:56
    
Good first question! –  slm Dec 22 '12 at 4:15
    
Are you running a firewall? What OS is this? Assuming Linux, which distro? –  slm Dec 22 '12 at 4:16
    
Try to run master on another port like 2525. –  user128296 Dec 22 '12 at 12:47
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you can use tcptraceroute myserverip 25 to see where packets are dropped –  Eric DANNIELOU Dec 24 '12 at 14:43
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1 Answer

There is no such feature in Postfix. In really rare circumstances this may be possible, but then your Postfix would be under very heavy load. Heavy load means hundreds of mails per second.

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