Sorry for the bump in this thread, but I think this bump is better than opening another thread.
I migrated my server to another VPS a month and a half ago, today I just realized that I was not receiving emails outside of the domain itself... investigating I have seen that the problem lies in the blocking by zen.spamhaus.org.
On their own website there is an IP check, both ipv4 and ipv6 indicate that they are ok, but until I have removed reject_rbl_client zen.spamhaus.org I have not received emails from outside the domain.
And it is exactly the same as described in this thread:
The problem only occurs when I build the mail server with the option: AntiSpam/AntiVirus (recommended).
Tomorrow I will contact zen.spamhaus.org to see what solution they indicate according to what my colleague ccsinteractive says.
Greetings.