When MTA (Mail Transport Agent) such as Sendmail and Exim attempts to relay and transmits email through the mail server while receiving mail from external address to local account, the mail will not get delivered and been rejected with the one or all of following errors in the maillog log file.
2007-08-05 08:08:88 H=(example.com) [18.104.22.168] sender verify defer for <firstname.lastname@example.org>: lowest numbered MX record points to local host
2007-08-05 08:08:88 H=(example.com) [22.214.171.124] F=<email@example.com> temporarily rejected RCPT firstname.lastname@example.org: Could not complete sender verify
2007-08-05 08:08:88 H=mail.example.com [126.96.36.199] F=<email@example.com> rejected RCPT <firstname.lastname@example.org>: relay not permitted
The cause for the problem can either be because that the MX DNS record for the domain zone of destination recipient (local host) is wrong, or because (most likely reason anyway) that during the verify sender and recipient condition in RCPT ACL phase, the example.com is not recognized as a local domain, nor is the remote host allowed to relay.
To solve the issue, check whether the domain MX record in DNS name servers to verify that it’s pointing to the correct mail server. If MX record is correct, ensure that the destination domain is properly designated as the local-domain to allow Sendmail or Exim to know that they suppose to receive the mail without querying DNS again. To do so, insert the affected domain to the server’s domain lists into the following config file, depending on your flavor of Linux or Windows OS, and which MTA you’re using.