Do I really have to keep repeating “DNS problem”?
[joe@kate data]$ host customermail.gr
Host customermail.gr not found: 3(NXDOMAIN)
[joe@kate data]$ host -t MX customermail.gr
Host customermail.gr not found: 3(NXDOMAIN)
[joe@kate data]$
Obviously there is no customermail.gr, it’s just a silly name i put so i don’t share publicly customers’ domains 
gerstavros@gerstavros:~$ host li***.gr
li***.gr has address 46.*.*.*
li***.gr mail is handled by 10 shared.s***e.gr.
After all, they are public. And when your customers have created them, they probably want others to visit them as well. Or?
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Is it an answer that solves the problem of the topic? The company that cooperates with me, doesn’t want the server’s and domains to be shown in forums, is that something weird?
For future reference, when you wish to quote log entries but hide the actual domain name, the convention is to use domain.tld, in this and other forums, to indicate to the community that you have used a placeholder instead of the actual domain.
Don’t use clientdomain.gr or something arbitrary like that for the domain name placeholder.
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It can complicate things when trouble shooting. Just a domain by itself, if meant to be public, doesn’t really pose any additional risk by being posted here. You can post it and then change it a few hours later if you are worried about it.
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if your trying to get help to troubleshoot an issue it does help having the domain name, so others can test issues against the real domain. I don’t agree with adding a domain to a forum to advertise it. It could be said that adding a domain name/IP address to a forum would open a door to a spammer/hacker but if the system admin has hardened the servers security all attempts to breach it will be thwarted, so perhaps you don’t think you have secured the server enough and feel there is still a backdoor into the server or the domains are for private consumption only. That said if you have registered a domain that record is in the public domain so anyone can find it if they wish and attempt to exploit it. So I guess it could be classed as weird
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It is fine not to share the specific domain. But, it should be clear that’s what’s happening.
I continue to say that if you don’t have log entries, the problem is before the MTA, either DNS, firewall (not on the system, the default Virtualmin created firewall opens all necessary ports, I mean at your hosting provider), or Postfix isn’t running.
So, check DNS (the MX record for the domain, the A record for the name(s) the MX record points to), and check to be sure you can connect to port 25 on the server from outside the server (via telnet or some mailchecker website).
Yes. Real attacks on a given domain will occur, regardless of whether someone publishes its domain name on the forum or not. My personal experience is that the first unauthorized attempt to login to root takes place within 3 hours. And that on a domain that was not yet on the web. After 3 hours from connecting server with public address to i-net.
But for anyone who wants to help, it’s just a brake, because more or less all that’s left is fortune telling from a crystal ball. Then maybe the trivial problem is solved sometimes even in 11 days. Or more.