This plugin checks that the domain used in MAIL FROM is resolvable to an MX record.
This plugin uses the INI-style file format and accepts the following options:
Maximum limit in seconds for queries to complete. If the timeout is reached a TEMPFAIL is returned to the client.
Allow MX records that return IP addresses instead of hostnames. This is not allowed as per the RFC, but some MTAs allow it.
Return DENY and reject the command if no MX record is found. Otherwise a DENYSOFT (TEMPFAIL) is returned and the client will retry later.
DNS errors always return DENYSOFT, so this should be safe to enable.