Are you sending through a Yahoo account? If so, you may be able to fix it by changing the preference mail.strictly_mime to true.

https://support.mozilla.org/en-US/questions/1206273#answer-1080798

Thanks for the comment but I only send my mails directly through T 'Bird. I have Yahoo and Gmail accounts but do not use either for outgoing mail nor incoming, just T'Bird and have been using it since the end of Netscape

So, you have no Yahoo accounts set up in TB, or accounts that use Yahoo for mail service such as Verizon, Rogers etc.? Does making the preference switch affect what recipients see?

Also, check under Tools/Options/Display/Formatting/Advanced that Text Encoding for Outgoing Mail is Unicode UTF-8.

Choosing Tools/Option/Display/Formatting/Advanced shows UTF-8. I just sent a test email to a friend who notes the problem still exists.

OK, what happens if you change the preference to true?

Sorry, I don't know what "preference " is or where its located

Tools/Options/Advanced/General/Config. editor, double-click the preference mail.strictly_mime to true, OK to close Options.

Thanks. When I go to / Config.editor I get a warning that making changes there could Void my warranty. Is i safe to make that change ? I'm not tech savvy , just a casual surfer and emailer.

That warning is unnecessarily alarmist, and there is no 'warranty', so go ahead and make the change - which is easily reversed. In most cases it's better to make such edits through Options or Account Settings, when possible, but in this case you have to go into Config. editor.