Issue Really strange problem – I can't use my SMTP server intermittently

[carlsson]

Server operating system version

Ubuntu 20.04

Plesk version and microupdate number

Version 18.0.58 Update #2

Today was the third time I experienced the following, and with different networks:

- Suddenly I can't connect to the SMTP server.
- No devices on the same network can use that SMTP server.
- Connecting to another network (like my cellular) works normally.
- I have tried different accounts, all is on the same Plesk and behaves identically.
- Waiting an hour and the problem vanish.

The first time I noticed this behavior was last week when I helped my brother in law setting up his copier. That involved a lot of testing because it didn't work at the first attempt, and after a while SMTP was blocked. Connecting to my cellular showed no problems of sending emails.

Second time was during this weekend, also after a lot of testing, and just now it happened again, also after testing (see my other thread).

When connecting to my cellular it works as it should.

I get the feeling that my Plesk Mail server bans my IP during a period. I can't find any info about this in Plesk admin though, and my IP is not mentioned in the IP address banning logs... I whitelisted my IP to test, but there was no difference in behavior (my IP is a DHCP from my ISP).

The only common factor is my mail server. Everything else works.


Any ideas what this is all about!?

 

[Kaspar]

- Suddenly I can't connect to the SMTP server.
- No devices on the same network can use that SMTP server.
- Connecting to another network (like my cellular) works normally.
- I have tried different accounts, all is on the same Plesk and behaves identically.
- Waiting an hour and the problem vanish.

These symptoms perfectly fit a fail2ban IP ban. As these are temporary and can be service (port) specific.

If you haven't already, you could whitelist your IP. (Go to Tools & Settings > IP Address Banning (fail2ban) and click the Trusted IP Addresses tab).

If I had to take a guess, I would say that the banning is probably caused by the issue described in your other thread. But I it could caused be something different too.

 

[carlsson]

Thanks Kaspar, I agree, it must have something to do with fail2ban. But why can't I see my IP (or subnet) in the logs, that's really confusing...?
I did a whitelisting of my specific IP during the latest ban. Maybe it didn't help because it was an ongoing issue. Or maybe I need to whitelist my subnet.
I'll search the logs closer next time if it happens.