MJ2 causing greylisting delays

Hi MJ2 folks,

   Please see this message I received today...

---------- Forwarded Message -----------

Hi Alex,

  we've just noticed one issue with emails sent from your listserver.
Every email has a different envelope-from sender, and as such
is always delayed 15 minutes by the greylist.

  Here it can be seen:

Received: from example.com ([24.205.141.134] helo=gouda.example.com)
        by saus.something.sk with esmtp (Exim 4.60 (FreeBSD))
        (envelope-from <[email protected]>)

Wouldn't it be possible to change the sender just to "some-list-owner"
and drop the message number (+M...) part?

------- End of Forwarded Message -------

Any ideas? I'm guessing the +M### is mandatory?

Thanks,
Alex