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.
Yes, I think that's it. I only started seeing this a month or two ago. I'm inclined to believe some of the new mobile phone email apps that "everyone" are using these days are responsible for this. I'll start watching my email replies that show these characters and see if I can identify a culprit app.
I did check my Thunderbird email setting and it's set on Unicode.
If you're sending in Unicode, then it's possible/likely that the people who are seeing the strange characters are using a different character set on their client, in which case to get rid of the strange characters they'll need to set their clients to receive in Unicode.
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