Update: Status bar shows receiving multiple email messages, but only shows the referenced specific recurring email again and then error pops up again??

What happens if you open the account in webmail, delete the offending message, and empty the webmail Trash and Spam/Bulk folders?

Deleted offending message as suggested, to no avail. Do not see that email any longer, but now get the sane error message as stated above. Status bar says I am receiving multiple messages, but nothing shows up and the error pops up each time I retry. Other suggestions?

Avast user? https://support.mozilla.org/km/questions/1429446

No, and tried pausing all antivirus programs, still same error. Continues to download all messages (based on message count I see on task bar) from server but Inbox says 0KB and shows no messages in folder properties (repair folder appears to have no impact either).

Whatever AV you use, it's best to test with Windows running in safe mode, instead of 'pausing' AV. Safe mode bypasses startup apps like AV.

Also, do a search on the exact error message "Unable to write... copy the mailbox".

Thanks for suggestions. Appears to have corrected itself after overnight shut down? I did delete many older emails from my webmail inbox and also reloaded TB 140. I can't help thinking that part of my issue may have been my computer synching to OneDrive got "stuck", but in any case all seems to be working today...

I have multiple email accounts on Thunderbird. I noticed one of the main accounts had not downloaded any email since 9/2/2025. When I run "get messages", messages would attempt to download, but fail after 6 to 10 messages with the "unable to write...." error. I knew it wasn't a disk space issue, with 400+Gb of disk space available, and all the other email accounts (6 each) were up to date with their downloads. After reading the suggestions above, I figured the trouble had to be with a specific message. I went to my web mail, and moved all the recent messages (since 9/2/2025) to a holding folder, out of the Inbox. I ran "get messages"  : received "no new messages", This was correct. I started moving daily message groups into the inbox, and then ran a "get message" on that daily group of messages. I found two days with a message which would cause the failure. The message didn't look bad, or corrupted. It looked "normal" on the web mail. I deleted the possible offending messages. . I then was able to run "get messages" on all subsequent daily message groups with no problem when they were moved into the inbox folder. I'll keep an eye on this issue in the future, and not let the problem persist for days, if it occurs again. . Thanks for the comments above.

I have multiple email accounts on Thunderbird. I noticed one of the main accounts had not downloaded any email since 9/2/2025. When I run "get messages", messages would attempt to download, but fail after 6 to 10 messages with the "unable to write...." error. I knew it wasn't a disk space issue, with 400+Gb of disk space available, and all the other email accounts (6 each) were up to date with their downloads. After reading the suggestions above, I figured the trouble had to be with a specific message. I went to my web mail, and moved all the recent messages (since 9/2/2025) to a holding folder, out of the Inbox. I ran "get messages"  : received "no new messages", This was correct. I started moving daily message groups into the inbox, and then ran a "get message" on that daily group of messages. I found two days with a message which would cause the failure. The message didn't look bad, or corrupted. It looked "normal" on the web mail. I deleted the possible offending messages.

Thanks for reporting this ...

In the future, please keep the offending message and file a bug report https://bugzilla.mozilla.org/enter_bug.cgi?product=thunderbird

Also, in general best to create your own support question https://support.mozilla.org/en-US/questions/new/thunderbird