Provide for insertion of RCPT-TO (envelope sender) in messages
When using an email filter in the mail flow to Bcc a message to an alternate destination, the envelope sender (RCPT-To) is understandably changed to the new Bcc destination. It would be nice to be able to include the original envelope destination in, e.g. in an X-Original-To header.
This could be done with a dedicated option, or by allowing variable substitution in the "add a header" filter action.