You see a series of Unsubscribe Email activities, roughly 5 minutes apart, covering more or less all the emails someone received:
Each one’s Details show they used the List-Unsubscribe header:
Turns out that’s standard behavior of the newish Manage Subscriptions feature in consumer Gmail.
When you go to Manage Subscriptions, Gmail shows you addresses that’ve sent enough marketing email to be trusted, which it then considers a “Subscription”:
Clicking on a Subscription shows emails from that sender that are still in your mailbox:
If you click Unsubscribe on the Subscriptions tab, Gmail visits all the List-Unsubscribe URLs in sequence, waiting 5 minutes between URLs to ensure it doesn’t exceed rate limits. This is mentioned, in a dumbed-down way, in the docs:
Since every Marketo email has a unique List-Unsubscribe header — if it didn’t, we couldn’t track Unsubscribe Email at the asset level! — all emails are individually unsubscribed. This can mess with reporting, so look out for it.
Note neither side is misbehaving, although it does suggest a new SMTP header like List-Unsubscribe-All would be useful. (Good luck getting that through standards committees!)
