Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

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

Does the problem go away?

Hey thanks a lot for the suggestion. Can you explain why you think this may work? What would safe mode allow that regular operation wouldn't that may be causing this issue do you think? I guess it's not so difficult to try but curious as to method :)

Chances are your problem is caused by anti-virus software. Windows safe mode disables anti-virus software.

Thanks again for the input. I don't have any third party AV software installed on this laptop, just whatever comes included with W10.

But if it's happening only on these bigger mails and the mails are certainly not infected with any viruses, and hundreds of other mails successfully transfer, do you still think it could be AV and that this could resolve the issue?

Windows safe mode is not going to fix your problem. It is an attempt to determine whether anti-virus is actually the culprit.

Thanks again for your continued support. I managed to start windows in safe mode with networking and did the same with thunderbird and started to transfer around 30 of the problematic mails. Before it would fail after the 2nd or 3rd, and it was at around 9/30 with no issues but then again, it gave the server time out :(

I'm thinking it may be Gmail related as opposed to anti virus issues? Is it possible to allow more time for it before it decides it's a failed transfer?

It's a big pity there's no retry option, and it just fails and cancels the transfer without you even realising it if you're not at the computer.

Email servers are not really made for mass transactions, and I believe Google are throttling transactions once a certain data volume threshold per day has been reached. I have no further details though. So you may try not to move everything in one step, but split the messages into multiple smaller batches.

Thanks again for the reply. This is what I'm thinking, but then again, it allows more mails to be transferred when you try again, so it's not like I've exceeded a daily allowance or anything.

When dealing with over 50,000 emails and it's choking on a small batch of 32, I'm not sure what my best option is ;(

Why do you need to have the old POP mail on the IMAP server anyway? If you have multiple computers accessing the account, it looks like it would be a lot faster and more reliable to simply copy the POP messages into the Local Folders of each computer. Granted, this won't make the mail available on a cellphone, but they are usually configured to store only a limited number of messages, such as a week's worth or a month's.

Thanks for the input. This isn't really about having access from multiple computers, it's more about always knowing that I have every email backed up to the server, as well as locally. I've been using POP for over 10 years so have all that mail to back up to the server so I'm not relying on local backups. I've had a few cases where I've had local files corrupted and I just can't take the risk of keeping it local only,

I think I may have found the issue here. After arranging my emails by size, selecting large chunks of mails low in file size, they transfer right over and no time outs yet.

Any time it has timed out previously, it seemed to be on mails with large attachments. I have some mails with attachments over 50mb and these have failed numerous times when trying to move to the Gmail IMAP folder.

Is there any way to get these larger emails over to Gmail I wonder?

I still have thousands more to go so will be busy transferring the smaller mails over before I know if it's working for sure.

Well I left about 20,000 emails transferring before I went to sleep and woke up to a crash report from Thunderbird ;( I don't know why it crashed, but I see about 15,000 of the mails are now in the "move to" folder, but as has happened before, when the transfer doesn't complete and is interrupted, the same mails from the "move from" folder, don't disappear, so I still have the same amount in the "move from" folder.

If the transfer completes successfully, the mails from the "move from" folder that are moved to the "move to folder", disappear, leaving only the mails left to be transferred in the "move from" folder, which is much nicer as you can just continue on.

For peace of mind, I still like to transfer the mails that appear to have transferred so it does it as it's meant to. I'm not sure if this is necessary, so I'll probably transfer the same 15,000 mails again ;( Google doesn't seem to duplicate mails it already has which is something at least.

I was doing some more searching on this last night and found this program which seems to have some good reports and reviews: https://www.ghacks.net/2011/01/07/google-webmail-filer-for-thunderbird/

From the description it seems to be what I'm looking for, but I'm a bit apprehensive to use it as I'm not sure how it will work given I'm half way through all transfers already so I'll probably just continue to do it manually, but maybe useful for anyone else reading this.

The developers website seems to be down though so I'm not sure if it's still safe to use or what and the discussion linked to above is over 3 years old.

The program is still available for download here: http://www.softpedia.com/get/Internet/E-mail/Mail-Utilities/Google-webMail-Filer-for-Thunderbird.shtml