The 'Sent' file in my 'Mail' folder exceeds 3 GB (3,224,360,399 bytes) and is not being read in the TB Mail window. How can I reduce the size of this file AND save messages??? 'Sent' message folder IS searchable!!!

What makes you think the size is impacting your ability to read this folder? Please post a screen shot?

Thank you, Wayne, for your thoughts. Screen shot attached.

You are on Windows 10.

Do alt-key > View > messages > all

Yes Windows 10

RE: Do alt-key > View > messages > all "View" menu item does not contain a "messages > all" option See attached

Ah, cool. "Messages" should be just below "Sort".

Something is definitely broken. If this is an imap account, right+click the folder, properties, and "Repair".

If that didn't help, start Thunderbird with Help > Troubleshoot Mode.

If that didn't help, start Thunderbird, click the folder, ctrl+shift+J and look for error lines

I agree something does seem broken I did the "repair: function on the Sent folder => no change It's a POP3 config Troubleshoot mode => no change start Thunderbird, click the folder, ctrl+shift+J and look for error lines => see attached Do not know what to make of Error Console information

Sent file is: 3,224,400,697 bytes

P.S. Glad to provide APPLICATION BASICS, however, this UI does not seem to permit uploading .txt files

Post the entire troubleshooting information at https://pastebin.mozilla.org/about/

• click new snippet
• change the expiry to 21days (the default 1 hour is probably going to see it deleted long before anyone looks at it)
• Click the paste snippet to complete the process.
• Post the resulting URL here.

It might also help to try the troubleshooting mode from the help menu, continue when prompted without changes as I have a feeling your issue might be a theme.