All three functions; TRUST SENDER, BLOCK SENDER, and UNTRUST SENDER in the webmail do not work on any emails where the RETURN-PATH: header is either Null <> or UTF-8 Encoded (i.e. =?utf?q?=3Cbounce-1234=5F=40bounce=2Fexct=2Epu?==?utf-8?q?domain=2Ecom=3E?=). No error is thrown, and the only way you can tell that something is wrong is that there is no green box confirmation in the lower-right stating that the action is successful. The Trusted Senders and Blocked Senders remain unchanged.

This is confirmed to occur in the most recent Build 8839 but seems to have been occurring for some time.

I understand why this is not working with Null <> values (once upon a time these did work with Null <> values and untold horrors were unleashed because of it), but it should be able to decode any encoded addresses and take action upon them it seems.

Not entirely sure if intended by design or a bug.