If you are absolutely certain the internet can connect to port 25 (i.e. you have tested it, not someone told you so), then you need to look in the mail log or the journal for the postfix service to see what happens when mail arrives.
i have updated the above message.
where is the mail log? /var/log/mail.log doesn’t exist on any of my systems
and journal gives nothing log regarding postfix or other related service
And Virtualmin still shows that emails are received on this user
but the folder has no mails received
This is why I use Rocky 
I already told you, if there is no maillog/mail.log, then you have to look in the journal for the postfix service. (journalctl -u postfix)
Rocky uses the journal for most things, too.
Webmin doesn’t list all logs nor does it know all the Journal units. Journal support is improving, but you still have to know a little bit about how to find stuff in the journal (but, that’s table stakes for being a Linux admin, today…that’s where most of your logs are, and you have to look at the logs!).
I haven’t bonded with the journal stuff when it comes to logging. It seems to be a FIFO system with a short memory. If rsyslog isn’t on the system when VM is installed is it a pain to add it?
That’s based on configuration and disk space. If it’s very short and you have plenty of disk space, it means something is logging like crazy and you should figure out what.
It’s just installing one package, but that won’t magically make all your old text logs appear again, I don’t think.
Rocky/Alma neither have enabled that module, at least on my systems
Yes, but nothing usefull exists there:
I had already also checked the whole journal log, not only about postfix, when sending an external email. There was no log regarding the email’s i was senting to the user
I see log files in webmin, not sure what you mean about module.
If you see nothing in the log (whether the journal or maillog), then it means mail isn’t reaching your server. DNS problem, probably.
Please explain me this, so i can understand what is the problem and fix it
I sent an email to the following address during the following time.
journal logs:
Feb 20 20:00:21 shared.server.gr dovecot[137037]: imap-login: Login: user=<info@customer.gr>, method=PLAIN, rip=::1, lip=::1, mpid=194683, secured,>
Feb 20 20:00:21 shared.server.gr dovecot[137037]: imap(info@customer.gr)<194683><gGLV+9MR4KcAAAAAAAAAAAAAAAAAAAAB>: Disconnected: Logged out in=93 >
Feb 20 20:01:24 shared.server.gr dovecot[137037]: imap-login: Login: user=<info@customer.gr>, method=PLAIN, rip=::1, lip=::1, mpid=194822, secured,>
Feb 20 20:01:24 shared.server.gr dovecot[137037]: imap(info@customer.gr)<194822><12ii/9MRWM8AAAAAAAAAAAAAAAAAAAAB>: Disconnected: Logged out in=93 >
Feb 20 20:01:35 shared.server.gr dovecot[137037]: imap-login: Login: user=<info@customer.gr>, method=PLAIN, rip=::1, lip=::1, mpid=194851, secured,>
Feb 20 20:01:35 shared.server.gr dovecot[137037]: imap(info@customer.gr)<194851><f3VIANQRqrkAAAAAAAAAAAAAAAAAAAAB>: Disconnected: Logged out in=414>
The login/disconnects are because i had open the roundcube on this email.
Nothing on Log regarding the email receive. BUT, as i have told other three times above, in user page of virtualmin, it says it received that time the email. But email is not in the user’s folder. This is not a problem of DNS or firewall or whatever is mentioned above. The user receives the email as mentioned always on the time of last mail received. So, where is the received mail?
This exact behavior i see on two different servers, the one is a simple VPS from hetzner.
Please explain me this behavior, so i can check what’s happening. What is this page’s code in virtualmin reads to show the time of last mail?
I Checked the code and saw that the “New mail received” variable $lastmod is just checking for changes in Maildir folder for changes, and it seems to be changed every time you press refresh on roundcube to check for new mails. So it’s irrelevant actually with mail receiving.
So, although firewall is configured properly, all ports are open, there are no mails received actually.
I still can’t see where to start again 
That’s Dovecot IMAP. Has nothing to do with mail being sent or received via SMTP.
yes. that was the only logs that time in journal (except some mariadb logs too that i deleted from text.
It seems mails don’t arrive in system at all. But that makes no sense as everything seems properly configured.
This message was created automatically by mail delivery system. THIS IS A WARNING MESSAGE ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE. The original message was received at Tue, 20 Feb 2024 19:46:34 +0100 from my@email.com [my@email.com] ----- The following addresses had fatal errors ----- [info@customermail.gr] Message will be retried for 4 more day(s)
Reporting-MTA: dns; mx.zohomail.eu Arrival-Date: Tue, 20 Feb 2024 20:01:29 +0200 Original-Recipient: rfc822; info@customermail.gr Final-Recipient: rfc822; info@customermail.gr Status: 421 Action: failed Last-Attempt-Date: 20 Feb 2024 18:46:34 GMT Diagnostic-Code: Problem communicating with the recipient server Remote-MTA: dns; shared.myserver.gr
Received:from [192.168.1.239] (adsl-19.37.6.116.tellas.gr [37.6.116.19]) by mx.zoho.eu with SMTPS id 1708452090157430.20299995837604; Tue, 20 Feb 2024 19:01:30 +0100 (CET) Message-ID:<f3b9098d-1411-42ba-8218-ec52a6d0add4email.com> Date:Tue, 20 Feb 2024 20:01:29 +0200 From:=?UTF-8?B?zpPOtc+Bzr/Ou8+FzrzOrM+Ezr/PgiDOo8+EzrHPjc+Bzr/PgiAoR2Vyc3Rh?= =?UTF-8?B?dnJvcy5jb20p?= my@email.com To:info@customermail.gr Subject:=?UTF-8?B?UmU6IM60zr/Ous65zrzOrg==?= Content-Type:text/plain; charset=UTF-8; format=flowed User-Agent:Mozilla Thunderbird