I am the administrator of innovation-matrix.com mail server running behind a public ip of 18.104.22.168
Recently two of our major customers failed to receive our company emails.
They are both using messagelabs.com services.
After looking at the SMTP server log files, we came accross the issue of
2017-06-22 22:31:51  1dNr7G-00093c-RO == email@example.com <JDurham@appliedrobotics.com> R=dkim_lookuphost T=dkim_remote_smtp defer (-44) H=cluster5a.us.messagelabs.com [22.214.171.124]:25: SMTP error from remote mail server after RCPT TO:<JDurham@appliedrobotics.com>: 421 Service Temporarily Unavailable
Please advice what can be the source of such an issue and if there was a penalty at this IP, please remove it from your databases.
Thanks in advance