Hi Stephen, First, let's try something simple. Right-click on your Sent folder, and select Compact.

Hi Chris. Thanks for your suggestion. I sent a 3.3 meg attachment. Unfortunately this did not work. Still getting "Copy Failed" What would be our next troubleshooting step? Thanks, Steve

Let's make sure you are subscribed to the Sent folder.

1. Right-click on the account, and select Subscribe.
2. Make sure that there is a check mark beside the Sent folder for that account.

If the problem still occurs, we'll need some more info about your Thunderbird setup.

1. Go to [=] > Help > Troubleshooting Information then click Copy text to Clipboard.
2. Go to https://pastebin.mozilla.org/ and go to Edit > Paste to paste the info from your Troubleshooting Information page.
3. In Firefox, click on the three dots to the right of the address, and select Copy Link.
4. Open a reply to this post, and go to Edit > Paste to paste the link to your troubleshooting information.

Hi Chris. I confirmed the "subscribe" was selected, and the check mark beside the SENT folder. Then, sent a 700k attachment...went thru fine. and saved to sent fine. Then, sent a 3 meg attachment..."copy failed", same problem.

Then, followed your instructions: Troubleshooting Information. Here's the link https://pastebin.mozilla.org/9091817

Hi Chris...one more thing. I noticed when I went to save an email as a draft or template, and that email has an attachment...same problem, the save to..feature times out and fails. Gosh...is there anything we can do to fix this? I really love Thunderbird but if it's unfixable I'm sunk!

Perhaps the Send Later extension is interfering? If you go to Help > Restart with Add-ons Disabled and test the issue, does the problem still occur? .

Hi Chris. In safe mode...still not saving to sent box. Error boxes, copy failed. Also, Connection to server timed out. box. See attached. Any thoughts?

Hi Chris. I removed the send later add-on and the calendar add-on. And sent a 3meg attachment...and still error saving message to Sent. and the strange error box...server timed out secureserver.net over at godaddy. Any other troubleshooting things we can try? I'm hoping!

Chris...smaller files go thru no problem and are saved to the sent file. Maybe there is some cap on the size of the attachment that's causing this??? Any ideas are welcome, Thanks, Steve

Let's try replacing the Sent folder.

1. Create a new temporary folder to hold your sent messages.
2. Move all messages from the Sent folder to the temporary folder.
3. Go to Tools > Account Settings.
4. There is a separate Server Settings panel for each one of your accounts. Select the one for the account where the problem is occurring.
5. Under Local Directory you should see the location on your PC where files for that account are stored. Copy/paste that location in Windows Explorer, then close Thunderbird (It's important that Thunderbird is not running when you execute the following).
6. There are two files for each mail folder (e.g. Sent and Sent.msf). Delete both files for the folder is giving you problems.
7. You can now start Thunderbird, and those files should be recreated by Thunderbird. Now you can move the messages back, and see if the problem still occurs.

Hi Chris. So a couple of things. I created a temp send folder...I "moved" the 5,600 messages to this folder. But when the action was completed only 500 message from 2013 are showing. The original Sent folder is now showing "0" messages!?! Also, underTools>Account Settings...I can not find a "Local Storage" panel.

There is a tab below all the accounts called Local Folders. In there I was able to see a directory:

C:\Users\Sttephen\AppData\Roaming\Thunderbird\Profiles\stw9r1ka.default\Mail\Local Folders

But...I have to go back to the "Mail" directory...one back...to see the Sent and Sent.msf files. So I did not delete yet...because I 'm not sure if at this point I've lost my sent messages?

Please advise, thanks, Steve

What anti virus product are you using?

Have you tried placing an exclusion in the anti virus for your Thunderbird profile folder?

I use ESET. I do not think I have an exclusion for the Thunderbird profile folder...if there is such a folder in ESET.

More importantly...after following Chris's advise... I've now lost most all my "Sent" folder messages!?!? As you can imagine...it's a bit distressing considering I use my email for business and refer to the Sent file for things I've sent customers...now can not find!?!? So...One problem begets another. Any suggestions on how and where my Sent messages went and can I retrieve them? (We can worry about the sending message with attachments later at this point)

Hi Stephen, Can you access your email using a web browser? If so, are the messages on the server?

Hi Chris. Yes...miracle of miracles...godaddy had my sent messages....we went back 5 days before the attempt to "move" to a new folder. And they had all the sent messages....I got them back! Thank goodness

Right now, it looks like the messages are on the server, but Thunderbird has had trouble connecting/syncing with the server. That's why when it tries to add a message to the Sent folder, it can't connect.

The question is: why isn't it connecting?

I think Matt was right to suspect your security software might be interfering. As a troubleshooting step, can you temporarily disable ESET, and test the issue with ESET disabled? We'll then know if that's the cause.

NOTE: Before you try the above, copy the messages in your web-version Sent folder to a temporary folder.