Assuming this is about Scarlet in Belgium, I saw no recent reports here.

What does Thunderbird report? What error do you see? Could there be you have no new mail? Is it a login issue? Can you send? Do you see new mail using webmail? Can you log in using another computer or device and what do you see there? Is logging in not an issue, but it’s like there are no new messages? Did you recently set up another device (such as a phone) using POP that would be able to delete new messages after retrieval?

Thunderbird does not report anything. There is no error indicated. I have a lot of new mails arriving. I see this on the webmail of Scarlet. I assume that logging in is not an issue because I can still see the old mails on Thunderbird, but as soon as I log in, it says 'connected with scarlet', but where normally the progress of receiving new mails is indicated, there is immediately a full blue line !

Are you using a pop or imap mail account?

pop mail account

Assumptions and Internet don’t get along very well. ;) It’s not uncommon to see the old messages in Thunderbird - after all it’s POP, and they’ve already been downloaded so there is no need to log in. You should however not see a blue line immediately without Thunderbird notifying how many messages (or none) were retieved. I’m not seeing a blue line either, but that may depend on the OS, or an add-on.

- (Basically, step 1: ) Does the same happen in Thunderbid’s Safe Mode? (An add-on could be interfering and causing the issue.) - Are you sure the messages are not in the Trash, possibly caused by some antivirus software or mail shield features? - In Thunderbird, do the Tools (menu, press Alt) > Add-ons > Get Add-ons page and the Help > What’s New page display? (If not, security software may block some type of access.) - In Thunderbird’s connection settings (Options > Advanced > Network & Disk Space tab > Settings button), is there a proxy set? (Set it to No proxy and uncheck "Use this proxy server for all protocols".) - Did your antivirus product (if any) recently update, or could it be unaware of the recent Thunderbird update? (Remove its entry from that software and make it "learn" about the new version, and make sure no new mail shielding feature is enabled by default.) - Have you set Thunderbird to leave messages on the server after retrieval? (I think so.)

You can enable the Activity Manager (in the Tools menu) to see what Thunderbird reports when it tries to fetch new messages. Each time it tries (automatically or manually), there should be an entry telling how may or no messages were downloaded. Make sure you see this proper activity, and that you see similar messages in the bottom status line.

When sure all of the above is fine (and only then): it might be possible that the popstate.dat file is out of sync, which usually results in messages getting downloaded multiple times since it did not keep up with downloading messages and writing the proper "already downloaded" status. However (and I have not seen this often), another possibility would be that your provider changed something that lead to the "X-UIDL: nnnnnn" headers of messages getting "renumbered" - see the 2nd or 3rd line when viewing the source of a message using Ctrl+U. In fact this happened in my setup a while ago, resulting in receiving a number of duplicate and older messages until a certain date. Theoretically, such renumbering could lead to identical numbers, causing new messages to no longer arrive as your mailbox already contains messages with those numbers. It’s just a thought and the odds are small, but nevertheless.

Either way, you can test for such an issue by simply moving the popstate.dat file out of your account’s profile folder\[profile_name]\Mail\[pop_server_name]\ folder to a safe place while Thunderbird is closed, and then start Thunderbird. That would lead to all messages on the server getting downloaded again, provided that you left the originals on the server according to the "Leave messages on server" setting in Thunderbird’s Account Settings. If that setting is not enabled, you would probably only see the missing messages getting downloaded.

If you want to avoid seeing duplicates, you may consider first moving all messages out of the Inbox into another folder in Thunderbird (i.e. make sure they are safely stored elsewhere) OR (my preference) using a file explorer by moving the contents of the Inbox, Inbox.msf and popstate.dat files from the folder just mentioned.

Note: if you have created other folders in Thunderbird for incoming messages besides the Inbox and messages were moved there (either by using message filters or manually), they will reside in other files than Inbox, and possibly even subfolders. That means you may also want to move these elsewhere in Thunderbird or in the explorer in addition to the Inbox.* files. Be sure not to delete the folders itself however, not in TB, not in the file explorer, as they might be part of or required by any message filters. Also, since redownloading the server’s Inbox files and deleting duplicates locally afterwards may cause the messages to get removed from the server too, be sure to uncheck "Until I delete them" in the Account Settings before deleting any duplicate copies, and then recheck the option when done. If not, deleting any of the 2 copies will probably delete them from the server if their UIDLs are (still?) similar.

Perhaps easier than the above: you could choose to add and launch Thunderbird with another profile (= pretending you are setting up another client), set it up as POP too, make sure to set TB to leave messages on the server as well before the first attempt to retrieve messages (i.e before entering the password), let all messages download, see if they download all, close Thunderbird and copy that profile’s Inbox, Inbox.msf and popstate.dat file back to the current profile. Basically this is similar as above, except that you will be doing it in a separate profile, and perhaps without setting up message filters and additonal folders. However, you can manually run the filters on the Inbox afterwards in the regular profile after copying back the Inbox.* and popstate.dat files. Advantages I could think of: being sure all messages will be downloaded once, resulting in a cleaner Inbox file, smaller file size, performance gain.

If the latter worked and the issue was resolved, you could compare some X-UIDLs from similar messages with those of the backup and report back to your email provider about the renumbering if that turned out to be true. Luckily my ISP admitted they changed this before asking if they did, after which they advised me to redownload all messages too.

But again, first make sure there is no other software on your machine capable of interfering with / intercepting email messages.