Recipient domains mangled for mailing lists

When I send to mulberry-dev@lists.mulberrymail.com, the "lists." part is getting stripped before the receiving server sees it, so the mail can't be delivered, and I get a bounce. The copy of the message in the bounce has been mangled to change "lists.mulberrymail.com" in the To: field to just "mulberrymail.com" and the receiving server says "no such user". (It's a different virtual mail server, so that's expected!) I can see the correct recipient domain in the envelope address in my outbound mail server log when it connects to smtp.comcast.net. So it looks like the Comcast server is changing that and rendering the message undeliverable.