If you disabled IPv6 in your centos installation then you can receive some bad problems. Some services needs ipv6 for default and if you disable ipv6 systemvide then these services not working correctly. For example Amavisd. If amavisd not working correctly you can’t send and receive mails in your server. Amavis trying to bind ipv6 address themself but you must fix this.
In first let’s look to mail logs:
tail -f /var/log/maillog
Probably you will see like that error log:
Jul 8 07:49:05 s2 postfix/error: 9858A62632: to=<email@example.com>, orig_to=<mailman>, relay=none, delay=544, delays=544/0.19/0/0, dsn=4.4.1, status=deferred (delivery temporarily suspended: connect to 127.0.0.1[127.0.0.1]:10024: Connection refused)
This means that mailman can’t connect to amavis. You must bind amavis to 127.0.0.1 (ipv4 address) and restart it.
nano /etc/amavisd/amavisd.conf $inet_socket_bind = '127.0.0.1'; # add this line to somewhere systemctl restart amavisd
After this time you can send and receive mails.