Support changing an envelope sender address to a subdomain to support DMARC

[trnubo]

When sending or relaying mail on behalf of a customer it would be useful to be able to rewrite the envelope sender to a subdomain that has been delegated to the email service provider (this container) so that DMARC compliance can be achieved.

This doc https://dmarcian.com/how-to-send-dmarc-compliant-email-on-behalf-of-others/ describes getting a subdomain delegated so the mail provider can configured DKIM and SPF independently of the root organisation domain.

It would be good if this container should rewrite the envelope sender to use a subdomain even if the From header is the root organisation domain.

eg. From: [email protected] rewrite to [email protected]