There's been some speculation that major email providers will stop parsing a List-Unsubscribe header if it contains a link rather than a mailto address, due to phishing and malware. I think this has happened on gmail now, cos there's no longer an unsubscribe button above our thank-you letters.
If true, we would have to reimplement unsubscribe as a mailto.
Our thank yous currently use $mailParams['List-Unsubscribe'] = "<mailto:{$verp['unsubscribe']}>";
ie an email not a link
- this would affect thank you emails, acoustic emails (they should have that in hand I think)