This took me 4 hours to suss out so I hope that this helps someone else.
If you set up a Relay Host in IBM Domino and are using authentication and it doesn’t work then read on.
The first sign of an issue is that the emails just stay in the
Enable SMTPCLIENTDEBUG=1   ( yes I know it says client but it works on the server )
tell router quit
load router
looking in the log you will now see a trace that says
SMTPClient: Attempting to Connect: Host
SMTPClient: Connection successful
SMTPClient: ReceiveResponse: 250-AUTH PLAIN LOGIN
SMTPClient: SMTP Authentication is required by local server. Username: -blank-
The -blank- bit is the problem because you have defined the user name in the configuration document but it is obviously not being used.
I can tell you that the issue is NOT encryption, is NOT ports, is NOT domino versions.
The problem IS that in this case the settings specify but this is actually redirected to
Changing the configured server address to causes the correct username and password to be used.
I am still having issues with Mandrill not being compatible with Domino in a subsequent step but I will blog about that if I can resolve that issue too.
A shout out to George E Marshall who provided the missing piece via a user forum 🙂