Sadly some US ISP's like AT&T and Bellsouth have started *corrupting* their customers' e-mail. If the you send using Encoding windows-1252 and includes for example special punctuation characters or a non-break space xA0, the ISP doesn't correctly interpret the the message as windows-1252 but as UTF-8. In UTF-8, xA0 is not valid and gets replaced by the so-called replacement character, � (0xEF 0xBF 0xBD).

Since the e-mail is still windows-1252 encoded, the recipient's client displays �.

Workarounds:

Set preference mail.strictly_mime to true.

Try this: 'Menu icon' > 'Options' > 'Display' > 'Formatting' tab click on 'Advanced' button

Set Unicode on both Outgoing and Incoming Check box "When possible, use the default text encoding in replies" click on OK

report back on : what text encoding you are using for incoming and outgoing mail. Was the checkbox When possible, use the default text encoding in replies" selected or not?

Wow! I just tested it and your solution(s) worked! Thank you very much! NO, the checkbox "when possible, ue the default text encoding in replies" was not checked. I then checked it.

I'm not sure where the "preference mail.strictly_mime is to set it to true, but doesn't matter..the other solutions fixed it! Thanks!!

this is really strange now! something has happened to change things. My settings are still the same and the upside down ? marks have gone away but for each time I use the backspace key, the ? becomes 2 question marks right side up (??) where I used the backspace key.

Applied the fix but now ? comes back right side up. What does mail.strictly_mime to true mean? and how to I do that?

• Menu icon > Options > Advanced > 'General' tab
• click on 'Config Editor' button
• In search type: mime
• This line: 'mail.strictly_mime'
• double click on the line to toggle the Value 'false' to a 'True'
• close window - top right X
• click on 'OK'

• Restart Thunderbird.

My menu doesn't exactly look the same as you listed above, for example I was looking for a "search" but finally figured out that it was "config" and then type in "mime", etc., etc. Thank you so much! I would not have ever figured out how to get to this point without this last bit of information. I tested it after I did the fix and now it does work completely!

You were correct - my mistake - I've updated the answer I gave, so if someone else locates it they will not be confused.