When your maillog on a CentOS/*EL machine looks like this:

mail postfix/smtpd[19345]: connect from unknown[XXX.YYY.ZZZ.QQQ]

The solution is really simple, just copy some resolve libs into postfix’s chroot folder:

mkdir /var/spool/postfix/lib64/
cp /lib64/*nss* /lib64/*reso* /var/spool/postfix/lib64/
service postfix restart

Now it should look like this:

Oct 31 09:00:34 mail postfix/smtpd[19424]: connect from mail-yh0-x245.google.com[2607:f8b0:4002:c01::245]

Enjoy!