Within the mailing list options, I set up the footer to show the subscriber's ID (which is a custom field, filled manually when subscribers are added) and an "Unsubscribe" link.  Adding the link is done by using a "custom variable" data field:  " #UnsubscribeLink# "

Here's an example of the actual URL behind the link for this client and domain, with an example subscriber:

http://[sub].[domain].[tld]/public/list.aspx?action=unsubscribe&email=[subscriber@theirdomain.tld]&doman=[clientdomain.tld]&list=[MailingListName@clientdomain.tld]

It seems to work just fine on desktop computers. But, several subscribers who tried to unsubscribe using the link reported being met with a login screen.  It took me a while to realize that this happens on mobile devices.  It appears that, on mobile devices, the unsubscribe URL gets truncated, causing the subscriber to be sent to the SmarterMail login screen, rather than an unsubscribe page/link.