Postfix not sending emails on slow networks

Hello all,

I have a virtualmin box with lots of domains (around 90). Since last week, several of my customers are complaning about email messages not being sent. The messages get stuck on the Outbox and are not sent.
It was working ok for me so I told them it should be a local problem, on the client side.

But I just did a test, using my PC connected to my cell phone’s 3G connection. It is a slow connection, but I can browse the internet with no problems. I tried to send 2 messages to the same domain my account is, and problem happend to me. Two very small email (about 10kb each) got stuck on the outbox and were not sent. I got the message “error (0x80042109) : 'Outlook cannot connect to your outgoing (SMTP)”.

Using the 3G connection, I can ping the server, browse normally, but emails are not sent.

When I reconnected to my office’s wi-fi connection, the messages were sent right away.

So, what can I do? Is there any error log to look into? Any postfix configuration I could tweak?

It used to work ok, but there are several different customers, from different ISPs, complaining about not sending emails emails, so I guess something has changed.

Please help, I’m about to loose some customers because of this problem.
Any advice will be greatly appreciated.



Well, I might start here… do you see any errors or warnings in the Postfix mail log whenever that occurs? Those are located in either /var/log/maillog or /var/log/mail.log.


Hi Eric,

No, there’s nothing on the logs.
It seems some ISPs are blocking port 25. I’ve tested using telnet and it works on port 110, 143, 80, but on port 25 it simply doesn’t connect.
So I configured Postfix to also listen on port 5525 , adding the line bellow no the file

5525 inet n - n - - smtpd

It worked, the messages were sent, and the clients using port 25 also kept working.

Now the question is, does the line above poses any security risk? All my configurations for port 25 will also be in effect on port 5525?

Thanks a lot

  • Rogerio