Try to use IP address if no MX record?

Currently, when trying to either use an IP address as the domain for an email address, or a domain without an MX record, it returns: `SMTP error 554: MX lookup error`
Could there be an option to try to send to the IP address directly?
(This seems to work in Outlook, for compatibility's sake?)

It would be useful for local email communications, maybe the mx-less domain part less, but it'd still be useful, it would make sense for this to be disabled by default.

Start by tracing the MX lookup error in the SMTP delivery path and identify where an email domain is resolved before connection. Review the existing configuration and delivery tests, then verify that an explicit direct-IP option is disabled by default and that IP-address or MX-less delivery follows the intended behavior.

索引モデルが issue の本文から書いたものです。