Mailman.BounceAddress is empty although the return-path contains a address.
Sorry for the confusion. The online reference documentation for BounceAddress will be updated to this:
The "return-path" address of the email to be used when the email is sent. Bounces (i.e. delivery status notifications, or DSN's) will go to this address.
Note: This is not the content of the "return-path" header for emails that are downloaded from a POP3 or IMAP server. The BounceAddress is the email address to be used in the process of sending the email via SMTP. (See the "SMTP Protocol in a Nutshell" ) The BounceAddress is the email address passed in the "MAIL FROM" SMTP command which becomes the "return-path" header in the email when received.