I have another laptop running Windows 7 with 32-bit version 115.8.1 and that works fine. Profiles are identical. Also runs ZoneAlarm as the antivirus tool.

Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Thanks. It did not solve the problem. However I have solved it as follows. I created a new folder in Thunderbird and copied all the emails in Inbox to that new folder. I then closed Thunderbird, deleted the Inbox and Inbox.msf files from the directory where I have stored emails. Then restarted Thunderbird and copied all the emails from the temporary folder where I have copied them to and it now works. Maybe the Inbox folder was corrupted? Don't know but did resolve the problem.

This is driving me nuts. I have another email address in my Thunderbird account and it too gets encountering the "unable to write the email to the mailbox" problem even while running with Zonealarm switched off. I deleted the inbox (and inbox.msf) files, restored the emails from a copied file, which incidentally took over an hour to copy about 450 emails (170Mb) using Thunderbird copy - and that is extraordinarily slow. I compared the all the mailbox settings to that which I have on my laptop and are identical. The laptop is exact same version of Thunderbird, has Zonealarm and is running Windows 7 Pro. The PC is the same (I even have it running 32 bit version rather than the 64 bit to be identical) though it runs Windows 10 Pro at 22H2 version.

The only reliable method to disable anti-virus software is to start Windows in safe mode as suggested above. I'd get rid of Zone Alarm altogether and just stick with Microsoft Defender.

In general, to avoid problems with anti-virus software, 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. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

I did try to run with Windows in Safe mode with Networking enabled but the networking would not start. Switching back to normal mode then network again would not start so had some work to do to get it back running.

Then the strangest thing happened after days of could not write the emails to the Inbox folder after about half an hour of sitting idle it suddenly decided to do just that and downloaded all my outstanding emails. I think the American phrase for that is "go figure"

Never had such an problem in over 20 years of various Windows systems with Zonealarm.

Subsequently Thunderbird updated to 115.9.0 and no idea if that has impacted the problem. Now running OK for two days and no problem writing to the Inbox folder.

Will leave it for a week or so to see if recurs and if not then will close this incident. Thanks christ1 for your guidance.

Well a few weeks on I thought that I had fixed the problem by backing off a couple of windows 10 patches (KB5035941 and KN5034466) as they were the most recent patches that I had applied. It ran fine for quite some time and yesterday came back on one of my mailboxes in Thunderbird. Two are fine one gets the unable to write to Inbox folder problem. Not sure now whether it is a Thunderbird problem or a Windows problem. So frustrating. I have a laptop running Windows 7 (Ultimate edition) with Thunderbird 115.9.0 and also with Zonealarm and that works find as it has done for many a year. Apart from re-installing Windows 10 I am running out of ideas.

So to try and overcome my frustration as one of my other email accounts on Thunderbird (yahoo this time) gave same problem so I installed Thunderbird Beta (120.0b5 - 64 bit) and it worked absolutely fine and quite a bit quicker. It didn't pick up all my folders from the non-beta account as I installed it in different directory.

I therefore deduce that my problem, which apparently is not unique, it is with the current releases of Thunderbird so would like to know when it is going to get fixed..

I thought that I had fixed the problem by backing off a couple of windows 10 patches (KB5035941 and KN5034466) as they were the most recent patches that I had applied.

I think that's a bad idea, and I doubt it's related to Thunderbird.

Never had such an problem in over 20 years of various Windows systems with Zonealarm.

Windows 10 does have a built-in firewall, there is absolutely no need for a 3rd-party firewall. The same goes for anti-virus software.

Apart from re-installing Windows 10 I am running out of ideas.

Did you create an exception for the Thunderbird profile folder in your anti-virus software as suggested earlier? https://support.mozilla.org/en-US/questions/1442510#answer-1643251

I installed Thunderbird Beta (120.0b5 - 64 bit) and it worked absolutely fine and quite a bit quicker.

The latest beta is 125.0b5. Running a beta version is not recommended for a production system, and I doubt that it is the solution for your problem.

It didn't pick up all my folders from the non-beta account as I installed it in different directory.

I'm not exactly sure what you did, but I'd hope you did create a new profile for the beta version. Otherwise you may not have an easy way back to the release version at a later time. https://support.mozilla.org/en-US/kb/thunderbird-beta

Be assured that the Beta version has a different profile and only has one Inbox. So far it is running fine, and fast.

The 115.9.0 version is back to having a solid failure of cannot write to the Inbox as above. I took your advice I just have the windows firewall and virus tool but that has made no difference and also took your advice and reapplied the Windows updates so am current.

Then I also copied my Inbox to a new folder, deleted all entries in old Inbox folder and ran a repair on the msf just to make sure it reflects the Inbox. Then downloaded all emails again .. well got as far as 20 and it gave up giving the usual error message that kicked off this incident.

Now yesterday I even got the unable to write my yahoo mail to the Inbox message on my laptop running Windows 7 and Thunderbird 115.9.0 and that is the first time I have EVER had a problem with Thunderbird on this device on which I have been using it since 2008!!!!!!

Other folks seem to have had similar problems but this one seems insurmountable Maybe version 125 of Thunderbird, which is my Beta version, will fix it but that could be a long wait.

I am getting to the point of giving up on Thunderbird.

Amy further ideas greatly appreciated.

I took your advice I just have the windows firewall and virus tool but that has made no difference and also took your advice and reapplied the Windows updates so am current.

Fine. Did you also create an exclusion in Windows Defender for the Thunderbird profile folder?

Yes, as you suggested earlier, I created a folder exclusion for Thunderbird in Users/...Appdata/.... Thunderbird and also a program exception for thunderbird.exe. What I get is it downloads 20 emails, then hangs, and then gives the mailbox folder error message. I do a reboot before a test just to make sure I pick up any defender changes.

Have you actually used chkdsk to check the physical disk for errors or failures. This is exactly the sort of thing I would expect to see if the file the folder is stored in is included in a sector that is having intermittent read/write failures.

You might also want to try using the windows volume defragmenter on the volume. I have found that in the process of running that, many disk level errors float to the top. The draw back is it is slow and really only suitable to run overnight on large disks

Thanks Matt,

I had done chkdsk, sfc /scannow, DISM as well as other disk health check tools. No errors indicated. As it is a SSD then defrag does nothing useful and guidance is not to use a defrag tool on SSDs anyway.

I have also copied the Inbox to a new folder, cleaned out the Inbox and tried to download emails .. TB says downloaded, e.g. 1 of 41, then hangs after about 20 with the famed error message.. A reboot and a restart does the same thing and never does it say that there are about 600 emails on the server so it is really screwed up.

So in desperation I backed out to 102.15.1 and all is now fine so I am now going to wait until TB gets to version 120 or later before I upgrade it. I also run Beta 120 and that is fine but would rather stick to released editions for my real "work.

You did prompt me to rerun the Crucial Storage Executive Device Self-Test and device has no errors, has 624GB free and firmware is up to date so I guess we can eliminate that as a cause. Supernova (TB release 115) isn't particularly super IMHO.

I don't know if it is any help, but I managed to sort this problem when it occurred to me yesterday after I had successfully downloaded two emails. It appeared in my Inbox on the PC, but my wife's inbox on the same account was fine.

I have duplicate Thunderbird set-ups on my laptop so I thought I'd still be able to get to my emails that way. When I (switched the laptop  on it downloaded outstanding emails up to the date of the problem then, after the same two emails I had downloaded on the PC, the same message appeared.

I concluded then that it must have had something to do the with either the two emails already dowloaded or those on the server that could not be. I logged onto the server, where there were three emails that had not been dowloaded so, having read and noted them, I deleted them from the server and also deleted the two emails that had been successfully downloaded that morning. This fixed the problem on both PCC and laptop and I concluded that there must have been something in the body of one of those five emails that had been the cause of the problem.