Change automatic email routing to use MX pointed to by nameservers rather than always using local MX

When "automatic" is selected on the Email Routing page for a domain, allow the server to determine whether the specified mail exchanger is remote or local based on the actual DNS that's in use for the domain instead of always using the local copy, which may not be in play.

When a domain is parked on an account and that domain's nameservers point externally (perhaps to the registrar), none of the local DNS for the domain is used. However, if the Email Routing settings is "Automatic", then mail sent to the domain from the local server is not properly routed according to the MX records that are actually in use for that domain. Instead, mail is sent to the exchanger listed in the local copy of the DNS for the domain even though it is not the actual MX record in use.

That is, if "Automatic" email routing is selected and a local copy of the DNS for a parked domain has an MX record that points to the local server, mail sent to the domain from the local server will be sent to the local server for processing (and likely discarded), rather than the actual mail exchanger in use for that domain, which is pointed to by the MX records stored externally where the nameservers point.

Since a domain may be parked by a customer such that the nameservers for the domain point externally, the "automatic" email routing setting should cause mail being sent from the local server to the domain to be delivered to the exchanger that is listed wherever the nameservers point, rather than always using the local copy of the MX records, which are not in effect in this scenario. This results in the automatic setting sending mail from the server to the domain to never be received by the actual mail exchanger in use for this domain.