I've implemented List-Unsubscribe (RFC 2369) for marketing emails we send. I am providing both an unsubscribe email address and an unsubscribe URL. An example of a generated header looks like this:
List-Unsubscribe: <mailto:[email protected]?subject=unsubscribe>, <https://myserver.com/[email protected]>
In the past few email campaigns we've done, it has worked great. There's only one problem. Sometimes we receive unsubscribe requests from email addresses we didn't actually send mail to. I think this happens when the user has multiple email addresses and the email we send is forwarded to some other destination. So we send to [email protected], but the recipient opens it at [email protected]. When they click the "Unsubscribe" link provided by their email client, it generates an email to us telling us to unsubscribe [email protected].
Sometimes we can find the intended address if the address we sent to was very similar, or if the user has a unique name, but sometimes it's impossible to determine which email address we should unsubscribe. That's frustrating because we know the user will be upset if they receive another email from us in the future.
I tried to fix this by adding a unique identifier to the subject line, so that a subject looks like unsubscribe_20934832034820348, but when we do that, email clients stop showing the Unsubscribe button. It's as if they will only show the Unsubscribe button if the subject line is exactly "unsubscribe".
I didn't see anything in the RFC about the subject line needing to take a particular form, and we are also taking care not to put the user's email address directly in the subject line. (It is a hashed combination of their email address and a portion of the original message, making it unique across all emails we send.)
Is there some sort of convention around this? If so, how can I reliably determine the original address we sent to when we receive unsubscribe emails?
It looks like there is no problem using this sort of subject line. However, it seems that each email client decides in its own proprietary way when and how to display the Unsubscribe button/link, and it does seem that that when you change from a simple "unsubscribe" to "unsubscribe" plus some unique identifier, some clients might subject you to some sort of test period before showing the link to users. In my testing, Gmail did not show me the link when sending small batches of test emails, but after I sent a large batch of emails, the link did start appearing, and I did indeed receive the generated unsubscribe mails properly.
I hope this helps someone out there.