March 2021:

The tandem@hobbes list is in the process of migrating to a Google group called tandemathobbes@googlegroups.com.

You do not need a gmail.com email account to subscribe (though that makes it very easy). Even if you do not use gmail, you can have a google account (tied to your other email address) which makes interacting with google groups a bit more convenient. But you can also just subscribe from your existing email address, in which case the list will be have much like it did before, though with no requirement to post in plain text. There is a digest mode.

If you have a google account (gmail or not) you can find the new group at https://groups.google.com/g/tandemathobbes and request a subscription.

If you do not have a google account you can request a mailing-list style subscription to the group by sending mail to the address tandemathobbes+subscribe at googlegroups.com (make that into an email address by changing " at " to an @ symbol)

Keep on tandeming!