I don't know if you got this sorted out. I've spent days on this and tried just about everything I can think of. My Email hosting provider has also been very helpful but with no success. K9Mail performs flawlessly on all other Android devices (phones and tablets) but not on the Nokia 7 Plus. It will fetch email fine, but outgoing mail looks as if it sends, appears in Sent folder, etc., but never arrives. I've had to abandon using it, which is a shame as I love it.
Let me know if you found any solutions.
John