hi David -thanks very much for getting back to me. i did follow this guide.
the YML formatting is correct - i pasted it in the message body and it removed the indentation (i should probably use code formatting tags but it wasn't clear if that's available in the text editor here). i know firsthand how picky YML files are because i had some formatting issues on earlier revisions of the file, which caused the canvas_init daemon to fail starting up. thanks to YAML Lint i think i have that issue solved.
the production log doesn't have very much, and i think the delayed job log has way way too many messages, but none of them directly relate to the mail server as far as i can recall.
i'll remove the quotes, but it's been confusing because i see a lot of versions of this file floating around that do have quotes, so i wasn't sure whether quotes were required or not. i suspect it probably won't change anything.
ideally i'd love to be able to test SMTP sending outside of Canvas first, to verify that that part at least works. but with the cloud server lacking a domain association, and no sendmail configured (i might have postfix but not sure) i don't know if this creates issues. what i have right now is a simple redirect on my my domain hoster for a subdomain that points to the cloud server's IP. no BIND that i'm aware of.
anyway, any further thoughts or advice are welcome. will update the post if there's any changes.