I can't access the mail server from inside the server

Community Support Mail
5

Hi, thanks for the extra details.

Because your server is behind a Mikrotik with NAT, the difference is exactly that mail.ugutec.com.ar resolves to your public IP (170.83.34.117), while server.ugutec.com.ar resolves locally to 127.0.0.1 from /etc/hosts.

When WordPress (or telnet) tries mail.ugutec.com.ar:587, the packets go out towards the public IP, hit the Mikrotik, and then need to come back in again. That’s hairpin NAT / NAT reflection. If it’s not enabled or not working, you’ll get the “connection timed out” behavior you saw.

That’s why adding mail.ugutec.com.ar to /etc/hosts works. It bypasses NAT and points the mail subdomain straight to localhost.

It’s not really a Debian 12 vs. Debian 11 change. More likely your old server had a different /etc/hosts setup, or your old Mikrotik rules allowed hairpin NAT and your new config doesn’t.

Options to fix it:

  • Keep the hosts entry (valid workaround, many admins do it).

  • Or set up split-horizon DNS so that internally, mail.ugutec.com.ar resolves to the server’s LAN IP instead of the public IP.

  • Or enable hairpin NAT on the Mikrotik so connections to the public IP loop back correctly.

That way WordPress can reach mail.ugutec.com.ar without hitting the NAT wall.

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