mail_from.is_resolvable

This plugin checks that the domain used in MAIL FROM is resolvable to an MX record.

Configuration

This plugin uses the INI-style file format and accepts the following options:

timeout

Default: 30

Maximum limit in seconds for queries to complete. If the timeout is reached a TEMPFAIL is returned to the client.
allow_mx_ip=[0|1]

Allow MX records that return IP addresses instead of hostnames. This is not allowed as per the RFC, but some MTAs allow it.
reject_no_mx=[0|1]

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.