Does MailStore Gateway need a MX record?
MailStore Gateway does act as a receiving SMTP server, however, MX records are actually optional. When there is no MX record, RFC 5321 §5.1 says the sending mail server should fall back to treating the hostname as if it had an implicit, self-referential MX record:
msgateway IN MX 0 msgateway.example.com.
In practice though, not every sending mail server implements that fallback, although it is a required behaviour. If you are using MailStore Gateway to receive journal-type messages from Exchange, Exchange Online or Google Workspace, no MX record is needed. If you are sending from another server, test the behaviour, or add the MX record of needed. I tend to create the record explicitly in my own environments for exactly that reason, and to make it obvious to anyone trying to figure out the setup later. It is not included in MailStore's documentation because adding the record introduces a potential failure point (a typo in the record) and another step to troubleshoot — a reasonable trade-off given how widely the implicit fallback is implemented.