It turns out that Sendy has a feature to prevent just this problem from happening, and I’ve now turned it on for all of our lists. The developer’s response to my report:
As part of email marketing’s best practice, Sendy includes a
List-unsubscribeheader in each email sent. ThisList-unsubscribeheader contains the unsubscribe link unique for each recipient and is used by email clients to display a native ‘Unsubscribe’ link at the top of the message if they choose to. Gmail for example displays a native unsubscribe link at the top of an email they think is a newsletter:The unsubscribe link points to the unsubscribe URL found in the
List-unsubscribeheader.Some ISPs use this
List-unsubscribeheader data to automatically unsubscribe a user when they think the email is spam. For example, corporate email servers are known to unsubscribe users with this data when they think the email is not important even before your email reaches the recipient. How email clients or ISPs use theList-unsubscribeheader is out of anyone’s control.The solution is to set your list’s ‘Unsubscribe setting’ to ‘Double opt-out’ so that another confirmation link needs to be clicked by the recipient in the unsubscribe page before they are being unsubscribed from the list.
