The problem may well be your anti-virus software. These are some generic suggestions 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

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. https://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Seems to be an erroneous popup only.

I doubt that. Hopefully you do have a good backup of your Thunderbird profile from before this issue started.

Also try R. click Inbox > Properties > click 'Repair folder' button, then R. click Inbox > Compact.

@christ1 Adding the antivirus scanning exception did not fix the issue. I'm using bog-standard Windows virus and threat protection. It does not add any add-ons to TBird. It has no settings for whether or not it scans incoming/outgoing email or attachments. Also, the popup about not being able to write shows up even when there is no new email downloaded. It is only intermittent, and I see no clues about what file TBird is complaining that it can't write (as far as I can tell, the only files it should be touching would be the inbox if there were new mail and popstate.dat (for my main account, some of the secondary accounts use IMAP). Also, as I have multiple email accounts, I can't tell which account TBird is complaining about.

@ed If you advise someone else of these same fixes, please reorder the suggestion so compaction is done first. Doing it your way caused thousands of deleted messages to reappear, which were a pain to delete again. Also, it didn't fix the issue.

Thank you both for taking the time to answer.

@ed If you advise someone else of these same fixes, please reorder the suggestion so compaction is done first. Doing it your way caused thousands of deleted messages to reappear, which were a pain to delete again. Also, it didn't fix the issue. Thank you both for taking the time to answer.

I don't know why that would have happened; I can't reproduce it.

hspindel said

@ed If you advise someone else of these same fixes, please reorder the suggestion so compaction is done first. Doing it your way caused thousands of deleted messages to reappear, which were a pain to delete again. Also, it didn't fix the issue. Thank you both for taking the time to answer.

I don't know why that would have happened; I can't reproduce it.

It happened because it had been several days since I compacted the mailbox. Repairing the folder undeletes everything that hasn't been compacted.

Ed said

hspindel said

@ed If you advise someone else of these same fixes, please reorder the suggestion so compaction is done first. Doing it your way caused thousands of deleted messages to reappear, which were a pain to delete again. Also, it didn't fix the issue. Thank you both for taking the time to answer.

I don't know why that would have happened; I can't reproduce it.

It happened because it had been several days since I compacted the mailbox. Repairing the folder undeletes everything that hasn't been compacted.

Honest question: did you really compile thousands of deleted messages in only 'several days'?

I'm going to have to disagree, because I tested it myself. I'm pretty sure what happened is that because the index was corrupted, the deleted messages were never really purged and they reappeared when the index was rebuilt. And I've not heard of that problem after recommending that procedure. But I do take note of it.

hspindel said

Ed said

hspindel said

@ed If you advise someone else of these same fixes, please reorder the suggestion so compaction is done first. Doing it your way caused thousands of deleted messages to reappear, which were a pain to delete again. Also, it didn't fix the issue. Thank you both for taking the time to answer.

I don't know why that would have happened; I can't reproduce it.

It happened because it had been several days since I compacted the mailbox. Repairing the folder undeletes everything that hasn't been compacted.

Honest question: did you really compile thousands of deleted messages in only 'several days'?

I'm going to have to disagree, because I tested it myself. I'm pretty sure what happened is that because the index was corrupted, the deleted messages were never really purged and they reappeared when the index was rebuilt. And I've not heard of that problem after recommending that procedure. But I do take note of it.

Yes, I had thousands of deleted messages after a few days. Typical of the spam load I get from having the same email address for 35 years. Fortunately, I have good ways to deal with that.

I don't know why you can't reproduce it. I've seen this behavior before. You are correct that the deleted messages were never purged because that's what compacting does, and without compacting they will resurrect if your repair the mailbox.

Have you seen this?

No, I hadn't seen that but thanks.

It's not applicable in my situation. I run my own local email server, and can see it has very little in it. Although you did give me an idea to check my secondary email sources, so thank you for that.

Also, when it happens see if anything relevant shows up in the Error Console:

- press Ctrl-Shift-J

- make sure 'Errors' is the only category selected

- if there are errors, R. click > Copy Message or Copy all Messages > paste here

In case anyone else reads this: the problem turned out to be an issue with one of my secondary email accounts which is almost never used. When I disabled the account, the problem went away. Don't know exactly what the cause was, though.

Thank you again to those who responded.

I'm still having this problem even though I've followed the various suggestions.

I'm still having this problem even though I've followed the various suggestions.

Try a new profile - https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles.

I'm reading though all the replies, as I have this trouble also. My system: Windows 10 I've used Thunderbird since vs 2.0, currently at Version 140.9.0esr I have six (6) active email accounts, and ten (10) inactive. Only one (1) of the active accounts started this trouble 4 days ago while I was away for the weekend. All other active accounts are downloading normally. All accounts use storage folders on the same drive on my PC, which has over 500 GB of available space. No accounts have disk space or download restrictions.

I have deleted emails and compacted multiple times, checked folder security and access settings, rebooted PC, and restarted Thunderbird. I will continue checking additional recommendations to clear the trouble.

I think I have found the issue with a message on AOL which Thunderbird doesn't like. I first moved all web mail messages in my inbox to a saved mail folder from the fist day this problem started (4/3/2026). I still was not able to download the messages from 4/4/2026 to today. So, then I moved the last five (5) days of messages on my web mail (AOL) to a saved mail folder. I had one message from today in my inbox web mail. I was able to download and write to the email folder with no problems. . Now I will move the messages from each day back to my inbox and attempt to download until I find the email message that is causing the trouble.

Thanks for the solution input and support.

I was able to find the "bad" email message, by selectively downloading email messages by each group of days. When I found a daily group that failed the download (3/4/2026), I started downloading the messages in groups of 10 or 12 until I was able to narrow the group down to smaller numbers of messages. The "bad" email was a seemingly normal message from Advance Auto Parts store. It looks fine in web mail, but obviously there is something that Thunderbird doesn't like about it.