Expose Header-From domain MX status

[ngocnha]

Stalwart already performs DNS lookups for SPF, DKIM, and DMARC during SMTP transaction.
Please expose whether the Header-From domain has valid MX records as a variable (env.sender.has_mx = true/false) that can be reused later in:

Sieve filtering

Report generation (DMARC / ARF)

Logging

This will allow:

Avoiding queuing undeliverable reports

Discarding emails from forged domains that can't accept bounces

Cleaner outbound traffic control

This data is already known during the initial lookup, it only needs to be preserved.

[mdecimus]

You already have access to the DMARC validation results from Sieve trusted scripts.