Queue 1 at 100 is fairly normal if indexing has not finished in your environment. Queue 5 continuously growing is likely due to this bug (currently not public).https://launchpad.support.sap.com/#/notes/2848886

You may be able to capture more information by getting the unencrypted traffic from the 3.1 system via wireshark/tcpdump, prior to it entering the stunnel proxy by logging the unencrypted port.BOBJ aliases the return code from the SMTP server to an i...

The official supportability policy is in this KBA:https://launchpad.support.sap.com/#/notes/1574766It can work - depending on how far apart versions are, and the complexity of content, success rates may vary, but there's no guarantee of compatibility...