Well I found a not too recent web posting on the web where the poster was having same problem with a Samsung. Apparently the outgoing port should be 25. Mine has always been set to 587 as per TT info. That has cured the problem, but for some reason when I go to settings it has reverted to 587 so has to be reset. No way to save the setting. Will go back to the Android forum and check.
OnePlus 5 review