Do you get some sort of error when you try and send mail that might provide some sort of clue as to why your attempt fails?

I did, but it's now moot. For no apparent reason TBird started sending messages using my Telus mail account. I suppose I did something, but I have no idea what it was, although I went through the various steps suggested by TBird help. I did alter various settings, with no apparent result: perhaps it took some time for changes to take effect. Thank you for you time and trouble.