Doesn't allow empty envelope sender

[baszoetekouw]

I'm trying to send a message with an empty envelope sender (i.e., retrun path). Changing the envelope sender using the impersonate plugin works well, but it doesn't allow empty values: if an empty value is set via setReturnPath() or addTextHeader('Return-Path'), it simple substitutes the message From header.

Empty envelope senders are particularly useful for automated messages (such as those typically generated by SwiftMailer, I would imagine). See, for example, section 3.3 of https://tools.ietf.org/html/rfc3834