How do you remove from Google Groups an old message authored by me, but that has an expired no longer used email?
To delete a message on a group on Google Groups, you should have access to that group and authenticate yourself as the poster or as a group moderator.
To identify yourself as the poster of a message the email used to post that message should be the primary or an alternative email address of the Google account used to access the group.
If you can't identify yourself as the poster, ask to a group moderator to remove that post. To do this on the mail page of the group click on the button Contact Owner. Note: This is an optional setting that may or may not enabled by the group owner.