If only some (but not all) of the recipients of your messages are having ths problem, then the most likely cause is that the text encoding they are using in their e-mail client inbox is not compatible with the characters contained in your messages.
You can check what character encoding you're sending your e-mails in by doing the following ...
With a Write window open in Thunderbird click Options > Text encoding which will open a list of the encoding options available. Default setting is usually Unicode, and if you have it set to that then most English speaking recipients should not have a problem with what you send.
If that's not the solution, then I'm afraid I'm out of ideas.
Edited by Gary R, 25 April 2018 - 11:24 PM.