Outlook describes my unsubscribe link as 'possibly unsafe' - why?

[Chris_Morris1]

Our company recently upgraded to Outlook 2016 (not 365). I'm testing a newsletter which will be sent internally as well to our external clients. The message includes an 'unsubscribe' link at the top (not my own, this is a function of Outlook) and the text when I review that link reads as follows: 

"It looks like this is a newsletter, but we aren't sure if it is safe to unsubscribe from it using the information provided by the sender. If you don't want to receive from this sender again, you can block it."

Does anyone have any information on why Outlook does not trust my unsubscribe? In this case, I'm using the default unsubscribe with a Marketo template I've updated code on, the unsubscribe is not hand-coded in my email. 

[SanfordWhiteman · Accepted solution]

Outlook is reading the List-Unsubscribe SMTP header, but doesn't consider it to have a trusted value.

Now, as for why it's untrusted, it could be that they don't believe Marketo users honor the Unsubscribed field, either because the field gets flipped back without user consent (I have seen this) or because emails are misleadingly classed as Operational (I have also, to a major degree, also seen this).

Or it could be that Outlook is simply too lame to understand that Marketo is sending a valid RFC 2369 List-Unsubscribe. Even though it contains a lot of numbers (who cares?) it's perfectly well-formatted.

More work would need to be done to figure out which of these is the case -- there's no deeper logging done by Outlook, just the conclusion "isUntrusted" after it pings the Unsubscribe add-in (part of Outlook/Exchange).

[SanfordWhiteman]

Update: even crafting other perfectly legit List-Unsubscribe headers (not Marketo-hosted, using <mailto:> and not the less-supported <http:>) my Outlook 2016 still classifies the link as untrusted!

I wonder if this is actually a misconfiguration on the Exchange side, as the Unsubscribe functionality is technically an Exchange app/add-in.

[Chris_Morris1]

Thank you for the information - you are beyond helpful, as usual.