Do you have an Anti-virus scanning incoming / outgoing emails or allowed to scan any opened thunderbird file or folder ?

Test: Restart Computer in 'Safe Mode with Networking'

Then, in Thunderbird, attempt to move an email with attachment.

If all goes ok, then you will know that the Anti-Virus is intervening and scanning, so it is taking too ,long and you end up with timeout.

In which case, try setting Thunderbird files and folders exempt from scanning.

Hi Toad-Hall, Thank you for your answer. I am on Mac OS (Mojave 10.14.4) and don't have an antivirus running (to my knowledge). Unfortunately, this would not apply to my issue.

How many messages are you trying to transfer between IMAP folders? Besides the fact that uploading to IMAP servers is likely to fail except for small numbers at a time, Google imposes bandwidth limits that could be relevant if you're moving large numbers with large attachments.

https://support.google.com/a/answer/1071518?hl=en

If the source account storage is large, you might be better off using the Takeout service to download the mail in mbox format, and then importing the mbox files to Local Folders in TB.

Thank you so much sfhowes. I took some time before answering you because I wanted to test out several things that you suggested.

- I was in the ~Gb range but it was same on a fresh new day.

- I had issues with mbox files because my thunderbird folders weren't local.

- What I ended up doing, which worked out is: (1) Export my emails as eml files. (2) Install the toolbox ImportExportTools for thunderbird, and import the whole bunch of eml files. It did go through with success. And after a few days for uploading everything I have finished importing everything in the gmail folders.

Thanks a lot. Although not exactly the same, your answer definitely oriented me to the right direction.

Manu: In my first attempt using Thunderbird to transfer emails from one Gmail account to another Gmail account, I experienced the exact same problem… and it took me several days (and nights!) to figure it out.

I believe that I have found the solution… here is what I did:

   1. Ensure the Thunderbird program is closed
   2. Open the web-based Gmail access program
   3. Sign into the account(s) you wish to transfer FROM
   4. Click on the gear-shaped “Settings” icon in the upper right corner
   5. Click on “See all settings”
   6. Click on “Forwarding and POP/IMAP”
   7. Under “IMAP access”, ensure that:  
       1. IMAP is enabled
       2. Auto-Expunge is off
       3. “Move the message to Trash” is selected
   8. Close the Gmail program
   9. Open the Thunderbird program

That’s it! After doing this, I have not encountered the time-out issue since!

I believe that the problem was due to the configuration setting that was instructing Gmail to archive everything being deleted, and the greater the file size, the greater the the delay in doing this, resulting in the time-outs when the maximum allowable amount was exceeded. My initial clue was that I noticed that the more emails I attempted to transfer, the greater the count of the Archives folder became. (Don't forget to double-check everything before you empty the trash!)

I hope this works for you. The reason I wrote back is because I was so deeply frustrated with the problem, and the solution was so quick and simple, I had to share it with you… ...there is no need for anyone to suffer! Ha ha!

:-)

Wishing you well. Mike San Diego, California USA

Manu:

After some deliberate experimentation, I have succeeded (?) in getting the timeout again.

Here are some things that seem related and tend to exacerbate the problem:

1. File size, particularly attachment file size
2. Conversational format (threaded discussions) – it helps if I expand the view of the compressed thread, which maps and shows the sub-hierarchy as multiple line email header entries, instead of keeping it all compressed and hidden within a single line email header entry
3. System resources: The more ongoing computer activities I have running concurrently, the worse the timeout error frequency
4. Most recently: Under the menu bar category Edit>Folder Properties>Repair Folder: I tried repairing the entire Gmail FROM account folder, and it has been churning away for over an hour, updating, downloading, re-indexing, etc. with an occasional pause, but hasn’t stopped once. It seemed like previously sometimes the system was getting hung, perhaps losing its place due to corrupt file system indexes… that sitting motionless until timeout is typical of a corrupt file index. This was also accompanied by the refusal to eliminate an email after successfully moving it to the trash, and similar glitchy behavior. The disk activity lights now are non-stop, and that wasn’t the case before when all the timeouts occurred. Maybe this will improve things. I’ll let you know, but it shows promise, and feels intuitively likely based on similar past experiences.

Later. Mike