Where do you see these? In received messages, or in your own compositions?

If in incoming messages, is this in all cases or just particular correspondents?

Usually, what you are seeing is symptomatic of inconsistent encoding, such as a mix of Unicode and traditional encodings. In theory an email message should declare the encoding it is using and the email client should respect and observe that declaration. A mismatch may indicate that somehow Thunderbird is set to ignore or override the encoding used in the message, or that whatever was used to write the massage is broken and makes a false or inaccurate declaration about the encoding to be used.

...In received messages, or in your own compositions?... in both, but in my composition they only (but not always) appear after I save (in Drafts). In recieved messages it happens haphazardly from different senders. ...is symptomatic of inconsistent encoding...YES I suspect something like this. But I don't understand the "encoding". Under Tools/options/display/formatting I set: Font=Verdana size 14 and under advanced: fonts for=latin and allow messages to use other fonts. Then under Character encoding I have: Outgoing Mail=Unicode(UTF-8) and Incoming Mail=Western(ISO-8859-1). After getting your message I believe this is probably wrong, but how was I to know. I have recently tried different fonts and the problem seems to occurr less often, but it depends on the numbers of punctuations used in the message. Mainly the apostrophe displays oddly, I see ’ for just one punctuation! Another one is … I think for exclamation. Most common is   for 2 spaces. Hope you can help, Thanks. syd50