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It would appear that the #UnsubscribeLink# variable used in SmarterMail Mailing Lists appears to no longer function as it should. When a recipient clicks on the link it takes them to a page that says "Your address has successfully been removed from the list" as it should, however, they still remain listed as a Subscriber and are never truly unsubscribed.
I don't know how long it has been this way, but I have verified that it is not working with version 13.0.5430. I can try installing the newest update later at the end of the week and verify that it hasn't been fixed since, but as there is no mention in the Release Notes I suspect this is an on-going issue.
Between this and not receiving Mailing Lists NDR bounce-backs when using a SmarterMail Gateway (which has been happening since version 8), it pretty much renders using Mailing Lists in SmarterMail inadvisable.