Right click the sent folder and select compact.

Thanks, but the problem persists.

See http://kb.mozillazine.org/Compacting_folders#Compacting_does_not_seem_to_work

Thanks. This does not solve the problem.

This does not solve the problem.

Please explain what 'this' means in detail. In other words, what exactly did you do?

I closed Thunderbird, made a backup, deleted the sent.msf files (that's where the problem is) & restarted Thunderbird. Then I sent myself a test message. It was sent & although I didn't get the error message, the message I sent does not appear in my sent folder. I tried sending another message & got the error messages again. When I click on the sent folder, the Windows blue circle spins continuously.

Try to start Windows in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01 - Win8 http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode# - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Thanks. I'll try this & post the result.

The problem is gone in safe mode. Now what?

What is your anti-virus software?

Windows safe mode disables anti-virus software, which is most likely the culprit. Some generic instructions:

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Doesn't that increase the risk of receiving viruses in or attached to e-mails?

No, it doesn't. If you receive a malicious attachment, it will happily sit in your Inbox doing nothing at all, unless you're stupid enough to save it to disk, and deliberately execute it (assuming it is an executable). And even then you're still protected by your anti-virus real-time scanner. Just don't save the attachment to your Thunderbird profile folder. Also, Thunderbird does not allow Javascript in email, so scripts cannot accidentally run.

In any case, use your brain. Don't open attachments from unknown senders. Don't open attachments you haven't asked for. Even messages with attachments from people you know may contain a malicious attachment, double check with the sender before opening it.

I don't do any of those things.

Thanks for the advice, I'll try your suggestion & report back.

That didn't work. I also tried disabling add-ons, but that didn't work either.

At this point, I am considering uninstalling Thunderbird, reinstalling and then restoring my backed-up profile. What do you think?

That didn't work.

You didn't explain what you did that didn't work. And you didn't mention what anti-virus software you use (despite being asked).

You may also ask in a support forum for your anti-virus software how to implement the changes suggested.

I am considering uninstalling Thunderbird, reinstalling and then restoring my backed-up profile. What do you think?

You'd just waste your time, as that wouldn't change a thing. Thunderbird keeps the program installation and the profile separate.

Sorry, I missed the question. I use Norton 360 Premier. I disabled the e-mail protection. I did not see an option to create a specific exception. I also checked the Norton site & saw nothing relevant.

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#Norton_Security

OK, I added profiles to the exclusions and restarted Thunderbird. I didn't get any error messages and sent a test message. The message is lited in the sent folder, but when I clicked on it, I got the message, "File not Found," following b y this: "The file mailbox:///C:/Users/Benjamin Zelermyer/AppData/Roaming/Thunderbird/Profiles/672avueb.default/Mail/pop.optonline.net/Sent?number=8 cannot be found. Please check the location and try again."

And now, going back to the sent folder, as I move the cursor over the messages, they disappear.

I'm going out for a while.

Thanks for trying

Verify with Windows Explorer the file 'Sent' exists. C:/Users/Benjamin Zelermyer/AppData/Roaming/Thunderbird/Profiles/672avueb.default/Mail/pop.optonline.net/Sent

Yes, it does (with msf extension.