FYI
I have been using DuoCircle’s outbound SMTP service for well over a week now and it’s working great. There is a nice “client area” where you can see everything about each message, the content, the attachments, etc (at first I was alarmed that they save messages but later relieved to see it’s only cached for what appears to be about 24 hours). I was able to do everything predicted in my message above. I will likely disable SMTP on my server.
Linode thinks my immediate issue was DKIM and not the UCEPROTECT scammers and I’m inclined to agree. But I’m not sure I see the point in running your own SMTP server anymore. I’m using the DuoCircle free plan at the moment. That looks like it’s set to expire in December. Then I’ll have to sign up for the $4 / month plan. That’s just fine.
Apparently there are SMTP servers that are free. Linode has a page about using Google’s for free. But for a variety of reasons I think I would rather pay a company who’s principal business is not advertising.