What is manual MX (/etc/manualmx)?

Question

What is manual MX (/etc/manualmx)?

Answer

ManualMX is a mail exchanger 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:

CONFIG_TEXT: example.com: external.mail.domain.tld

Going with the example above, even though my MX records may point to a specific server, the server will skip the MX lookup and instead deliver mail for example.com to external.mail.example.tld.

This feature is primarily used by the "Live Transfer" option in the Transfer Tool to proxy email from the old server to the new server.

Additional Information

How do I remove a domain from /etc/manualmx?

What is the Live Transfer option?