Changing the email addresses in headers

I'm trying to send out a newsletter and I can't seem to get enough control of the From and Reply to addresses in the email headers.

The WP install is on a server named, and apache is running under the account webdev.

I set up a newsletter, and I set the From address to When I send the newsletter, the From address is correct, but the server (running postfix) adds another header to the email.

Return-Path: <>

Unfortunately this is picked up by the SPF filter on Gmail and it invalidates the email, sending it to the Spam folder (although the From address is correct).

We have found a way to masquerade the domain in postfix so that the Return Path is correctly reported when sending out a Test/Preview newsletter, (Return-Path: <>:wink:, but incorrect when sending out an actual Newsletter (Return-Path: <>:wink:

However this is still causing problems. Is there a way to set the Return-Path header in the plugin (whether in admin or hardcoding it)? Or the Reply-To header?

We'd be happy to edit source files if this isn't currently available, and would like to suggest it as a feature for future releases of your product.

Any help would be much appreciated.