What is manual MX?
A manual MX is an MX redirection that lets you bypass the domain's MX lookup via the Domain Name System (DNS). The manual redirect entries are appended in the
/etc/manualmx file with the following format:
# grep example.com /etc/manualmx
Going with the example above, even though my MX records point to mail.example.com, the server will skip the MX lookup and deliver mail for example.com to external.mail.example.com.
You can remove or add a manual MX entry by following the procedure described in the following article: How do I add or remove a domain from /etc/manualmx?