You may see a warning like this in the Email Deliverability page.
The system sends “domain.tld”’s outgoing email from the “198.51.100.2” IP address. The only PTR value for this IP address must be “customhelo.domain.tld ”. This is the name that this server sends with SMTP’s “HELO” command to send “domain.tld”’s outgoing email.
This can occur if the custom helo contains a trailing space like so.
[root@cent7-90 ~]# grep "customhelo.domain.tld " /etc/mailhelo
Remove the trailing space from the /etc/mailhelo file.