This has the appearance of an antivirus or firewall software interfering with Thunderbird. That is, the setting is in a file for the account, popstate.dat, and it is not being updated at closure. Which means that thunderbird keeps retrieving the same message. You might look in profile folder in the Mail\<accountname> folder to see if there are multiple occurrences of the file (e.g., -1, -2, etc appended to name). If so, delete all but the one with highest number and rename that to popstate.dat (when thunderbird isn't running). As to why it is not being updated, I have no idea. And I could be wrong in my assessment, but that's what it appears to be.

I tried the "Clear Cache" in the Disk Space area of the Settings page.

It may have helped, will know more after opening and closing a few more times. It appears new messages were marked just for current day, not all the way back to 3/27.

There still seems to be some issue where another folder in the same account has 48 messages in it, but it still shows 49 on the folder list, and then you click the folder and it updates and shows 48, but then reverts to 49 the next time it opens.

Is it safe to delete the popstate.dat file to rebuild it, but is that relevant for an IMAP account?

My perception is that this is a POP account. If IMAP, please disregard my popstate.dat comment as it is irrelevant for IMAP. If IMAP, possibly changing settings at synchronization&storage pane may help.

It is IMAP, and it looks like clearing the cache with the button in the Settings, worked. Thanks.

Update, still an issue, but now the date is 4/16/25, all messages since 4/16/25 are downloaded as "new messages" every time I load the program.

Would you be able to send screenshot of your incoming server settings? That may offer some insight. Thank you

Server settings attached. Also, it looks like only the Inbox has this problem, any of the many other folders, segregated for various things, Important, News, Restaurants, Stores, whatever, only put the yellow 'new' message indicator by actual new messages.

My only remaining suggestion is to check synchronization&storage. I will see if I can locate someone with more IMAP experience to look at this.

Attached is the sync & storage settings. Always try to save everything locally. All of the folders in the Advanced popup are selected.

The computer is a bit slow at this point, an i5-4690, and I wonder if after closing Thunderbird some of the cleanup processing times out or something like that. A newer computer I'm also using doesn't have this problem with Thunderbird e-mail. But even the i5 machine didn't have this problem for years, it is recent.

Change check for messages to something 15 minutes or more and see if that helps. Hotmail simply used to ignore requests to connect at intervals of less than 15 minutes and the results you get on shorter time frames are often quite odd as it takes time for the downloads, and on consumer internet connections usually up to 4 times longer, for the uploads involved in synchronization. Add in a useless antivirus scan or four. Once to get the mail, once again to write it to disk and them a third to send the update back to the server that you have seen the message and another when the temp file is written to view the message and sometimes another scan because the read status has changed from unread to read. All in about 2 seconds usually, but I have had simple deletions take more than 10 minutes to be processed on the Hotmail servers (Hotmail, MSN, live and outlook as well of office365 are all interchangeable names for the Microsoft implementation of Exchange server they have on the web. It is all the same infrastructure, only different mappings depending on how much money you have handed over. Now slow things down with a VPN to confuse the routing of messages and it is a recipe for failure.

I have personally seen the Hotmail servers take more than 10 minutes to process a simple delete request via the POP protocol, which they chose to ignore and move the mail into a sub folder of the deleted mail folder. SO waiting to get your full new mail request a little longer will pay dividends as Microsoft tend to oversubscribe/overload their service. There is no profit in idle machines or factories.

To be clear here consumer internet plans are rarely symmetrical (business plans often are) and advertise download speeds because they are almost always much faster that uploads. Consumer internet being aimed at content consumption, not content creation. For example. Starlink advertise upload speeds between 5 and 25mbps and downloads of 30 - 100 mbps so when updates to things like IMAP require a lot of data transfer to the server things tend to happen much more slowly then folk anticipate, being used to streaming downloads the fair fly along.

Thanks for the reply, here are a few general thoughts:

1. Since I have two computers currently, the faster one is using Windows 11 and Thunderbird is not having this problem. 2. Even on the faster computer, it isn't Hotmail that seems slow, but rather the ISP e-mail accounts, which they must be throttling somehow, since even accounts that have no new activity, zero, will indicate 10-15 minutes processing time, but this is actually in the Microsoft Outlook program that I see that, Thunderbird really doesn't have a feedback like that in the Activity monitor popup. 3. I did set the automatic check time to 30 minutes, but honestly I don't really leave the program open for hours on end, it is usually opened, and then after it processes all the new messages, closed. But I have been waiting a while to close it thinking it needs to update something to fix this problem of not recording that it has already downloaded all the messages from 4/16 forward (this being an issue in the Inbox only, not the other folders.) 4. Today I decided to reset the Inbox folder and let it download all of the messages fresh. I'm old enough to remember slow internet going all the way back to dial-up, but with DSL now I can just let the program stay open until if finishes downloading all the messages and it won't take forever. The folder is 7gb in size. But it does take longer to download than it would take to download say a single program.exe file of the same size.

You have DSL now? What speeds?

Matt and David, could the combination of DSL and a large inbox cause this problem?

The speed is nothing special, these days, around 68mbps. But doubt that is the problem, the other computer has not had this issue in Thunderbird.

There must be some log setting/database entry in the Thunderbird code that is not setting, not recording the date through which messages have been downloaded.

I wish I could figure out how to get into the code, looked at it a few times and didn't follow through setting up the account in the depository or whatever it is that is required. Guess they need to have some security on it since as an open code project people can make changes, or perhaps all changes are reviewed by moderators first, not sure how that works.

That is twice now, I need to stop looking at this and responding when something is in the convection oven. Last night I burned a pizza, and just now a bagel burned.

Reset of the main Inbox folder fixed it. In addition to the hours it took to download the messages, it also had to create the Index for them again.

What is strange, is that now, two other random folders, neither of which have anywhere near as many messages in them, are downloading all messages every day as new if they are dated since 4/16/25.

So I can reset those two folders at some point, or just ignore it, but there must be some sort of flag in the code that is not being set for some reason.