This was precisely what I was looking for, thank you. Turns out auth was failing -- same UN/PW as for my mail clients, so.. ..looking into it, my MTA was accepting PLAIN auth, but not LOGIN auth. That's resolved now.

Once I get back to working with that instance, I suppose I'll go to my inbox -- where (you seem to be implying) it will show me errors from when the system sent me the initial authentication email? That seems like an odd location, but ok.

That sends me to my inbox here -- do you mean, on my own instance?

Also, the authorization process is rate-limited to 3 emails per hour, preventing me from using that to send an additional test email that way