Comment 1 for bug 266324

In a default installation, post will be determined to be
from a list member if the envelope sender or any of the
From:, Reply-To: or Sender: headers contain a member address.

You could send an off list message to the old address and
inspect the returned message to see if Return-Path: (the
envelope sender), From:, Reply-To: or Sender: contain the
old address. If so, accepting this would be expected behavior.

Note that in a situation like this, it may be preferable to
discuss the issue on <email address hidden> before
submitting a bug report.