From: https://support.mozilla.org/en-US/questions/1193450

The emails that I send with Thunderbird are clean and very professional when I send them. When my clients respond, their emails are also very clean and professional. However, the string of emails includes “�” after my sentences. These unusual characters also magically appear at the end of sentences in other individuals' emails in the earlier string of emails.

My clients tell me that the characters “�” appear in the email that they receive from me. I would like to know how to eliminate the unwanted addition of these characters.

You took me to a good place. I tried a variety of options and found that when I checked the checkbox 'When possible use the default text encoding in replies,' the problem went away. Thanks for getting me to the right place.