You don’t have an account with Mastodon. You have a Mastodon account with sdf.org. Their mailing list that sent the email and the Mastodon instance they run are probably separate systems. You can forward your email to membership@sdf.org and ask to be removed.
The clip Encarta included from this song will forever be burned into my brain.
https://youtu.be/vpA-uiUNHSg