Currently (Thunderbird 146; 12/2025), a problem is popping up on a number of computers that apparently affects Thunderbird POP3 profiles in particular, which have been “carried along” for many years and across several computer changes.
Here is our description together with a detailed problem solution.
The issue became noticeable through the following effect(s) in Thunderbird:
- No new emails are arriving; at the bottom in the status bar the connection attempt is shown as an endlessly long “connecting”, or is not displayed at all.
- The error message may appear: “Messages could not be written to the mailbox file. Make sure that the file system allows you write access and that sufficient disk space is available.”
- The antivirus program may report (either directly in a window or at least passively in its logs) a blocked file access to the file
c:\users\username\Appdata\Local\Temp\newmsg
What is the file newmsg?
A reassurance in advance: the existence of the file “Appdata\Local\Temp\newmsg” is not in itself an indication of a virus infection.
When retrieving newly received emails, Thunderbird creates files with the pattern “newmsg…” in the user’s temp directory, writes the incoming email into them, and then usually deletes them automatically.
Among other things, this allows antivirus programs to scan the message before it is transferred into Thunderbird’s actual mail storage (the INBOX file in the profile directory of the corresponding Thunderbird mailbox) and displayed to the user there.
Responsible for this is the useful option “Allow antivirus clients to quarantine individual incoming messages” in the Thunderbird settings, which should definitely be ENABLED and remain enabled.
This option apparently underwent a technical revision in Thunderbird version 146, because the description text in the above figure is significantly more detailed and specific than in previous versions.
In addition, the effect described in this article occurred particularly often in our customer base with the update to Thunderbird v146.
Which Thunderbird installations seem to be affected?
As far as we can see or guess a common denominator, the affected computers are those
- on which Thunderbird retrieves at least one mailbox via POP3
- on which an antivirus program is installed
- whose Thunderbird profiles have been copied over 1:1 for years, including when changing computers or upgrading the operating system
- whose Thunderbirds were “installed over” from 32‑bit to 64‑bit in recent years (because even then there were already isolated similar effects)
Important: The criteria mentioned above are
- NO reason to replace POP3 with IMAP!
- NO reason to uninstall the antivirus program!
- NO reason to disable the Thunderbird antivirus option shown!
These quick fixes circulating on the internet are not a real solution and lead to significantly worse privacy and an immensely higher security risk!
Solution – Step 1 – Check:
Close Thunderbird. (Also check in Task Manager to make sure no thunderbird.exe is still running.)
Temporarily disable the antivirus program for about 5 minutes (real‑time protection/file protection).
Start Thunderbird, check mail retrieval – are new emails now coming in, or is at least the inbox accessible, even if no new emails arrive?
(DO NOT click on any email attachments or links in emails! Antivirus is still disabled!)
For each mailbox in Thunderbird, do the following:
- File → Compact Folders (for the entire account).
- Right‑click on Inbox → Compact.
- Right‑click on Inbox → Properties → Repair Folder.
- Check: How many emails are in the inbox? Many users do not sort or clean up, and there are tens of thousands of emails in the inbox, which can make the INBOX file in the Thunderbird profile directory too large/complex or even corrupt.
- Check: How large is the INBOX file of the relevant mailbox in the Thunderbird profile directory? (Appdata\Roaming\Thunderbird\Profiles\meinprofil\Mail\meinpostfach\INBOX) If the file size is beyond about 15 GB, it is recommended to move older emails (for example: everything older than 1 year) into separate subfolders.
Now the basis has been created for further narrowing down the problem.
Solution – Step 2 – Interim consideration and user instruction:
One might assume that the cause of the problem is an infectious email that is about to be retrieved; Thunderbird places this in \Appdata\Local\Temp\newmsg so the antivirus program can scan it; the antivirus detects a threat and blocks file access to newmsg; Thunderbird then waits indefinitely for the file (i.e. that one email) to be released and responds with the logical but misleading message that there is an access problem with the mail store.
To confirm this, the antivirus program was configured so that it no longer performs automatic actions (“block file access to infectious files without prompting”), but instead asks the user – usually offering options such as “Block / Delete / Quarantine”. This led to a short‑term improvement, but a few hours later the affected users were back “on the mat” with the same problem.
The user should now first be patiently instructed that – if antivirus messages about a file “…\newmsg” pop up during mail retrieval – they must click the option “Delete file” or “Move to quarantine”.
By the way, briefly/testing‑wise renaming the INBOX file at the file system level in the Thunderbird profile directory does not help here; the problem is not caused by an old potentially infectious file in the inbox (in that case the antivirus would flag the file “INBOX.”), but by the retrieval of new emails.
Solution – Step 3 – maximally “clean” new setup of Thunderbird:
After the third case of this kind, the suspicion solidified that computers are mainly affected where the Thunderbird profile directories have been carried along over several computer changes or operating system changes since Thunderbird versions below v100 and have never been completely re‑set up.
Eureka – this allowed the problems to be solved permanently.
The path to success – version for IT professionals:
3.1) Start Thunderbird and note down the current Thunderbird configuration in detail:
- Export all calendars
(Right‑click on each calendar → Export → ics format)
_ - Export all address books
(Right‑click on each address book → Export → ldif format)
_ - For each configured mailbox, write down, screenshot or photograph:
- Email address, mailbox description, visible sender name
- Retrieval type (POP3 or IMAP), incoming mail server (hostname, port, encryption type), outgoing mail server (hostname, port, encryption type)
- Note/look up the mailbox password (if necessary in the Thunderbird settings: Privacy & Security → Saved Passwords)
- Signature text
- Junk/Spam handling settings
- Any custom folder assignments (Templates, Drafts, Sent)
- Any installed add‑ons/language packs and their settings
3.2) Close Thunderbird and use Task Manager to ensure that no thunderbird.exe is still running.
3.3) Rename the profile directoryC:\users\meinbenutzer\Appdata\Roaming\Thunderbird
toC:\users\meinbenutzer\Appdata\Roaming\ThunderbirdzzALT.
3.4) Uninstall Thunderbird and delete remaining directories
C:\Program Files\ThunderbirdC:\Program Files (x86)\ThunderbirdC:\ProgramData\ThunderbirdC:\users\meinuser\Appdata\Local\Thunderbird
as well as
- rename
C:\users\meinuser\Appdata\Roaming\Mozilla\ThunderbirdtoThunderbirdzzALT.
3.5) Restart the computer.
3.6) Download and install the latest 64‑bit version of Thunderbird.
3.7) Recreate all mailboxes (notes from step 1) with the Thunderbird „add new mail account“-feature
test them and configure all settings as noted in step 1.
3.8) Data transfer Part 1 – items exported in step 1:
- Address books (Tools → Import → From File → Address Books → ldif)
- Calendars (Tools → Import → From File → Calendar → ics)
3.9) Data transfer Part 2 – manual – rules, junk filter:
From the renamed Thunderbird‑ALT profile directory (see step 3), copy the following files into the corresponding new directory and overwrite them there:
msgFilterRules.dat– (message filters/triggers/rules); such a file exists for each mailbox.AppData\Roaming\Thunderbird\Profiles\profilname\Mail\postfach1‑n\AppData\Roaming\Thunderbird\Profiles\profilname\ImapMail\postfach1‑n\
training.dat– (junk filter training data); exists only once and applies to all mailboxes in the profile.AppData\Roaming\Thunderbird\Profiles\profilname\
If in step 1 the address books were forgotten, they can be found in the old Thunderbird profile as abook*.sql* and history.sql* (history contains the “collected addresses”).
3.10) Data transfer Part 3 – manually moving the old emails (only necessary for POP3 mailboxes):
The recommended, laborious but clean way is to place the old emails (i.e. mail files) into the Local Folders and move them from there within Thunderbird (“select emails, right‑click → Move To → [folder]”). In this way, potentially corrupt emails/mail files are revealed and all mail files are cleanly re‑initialized:
- Start Thunderbird.
- Under “Local Folders”, create a subfolder for each mailbox (for example named “postfach1”, “postfach2”, etc.), each containing a subfolder called “dummy”, so that the above‑mentioned folders “postfach1‑n” become directories in the file system.
- Close Thunderbird.
Now, from the old Thunderbird profile directory, proceed one mailbox at a time:
FromAppData\Roaming\ThunderbirdzzALT\Profiles\profilename\Mail\postfach1\
copy all contained files and subfolders WITHOUT *.msf and WITHOUT *.dat
toAppData\Roaming\Thunderbird\Profiles\profilename\Mail\LocalFolders\postfach1\
and analogously from
AppData\Roaming\ThunderbirdzzALT\Profiles\profilename\Mail\postfach2\
toAppData\Roaming\Thunderbird\Profiles\profilename\Mail\LocalFolders\postfach2\
and so on.
When finished, start Thunderbird. The “old” emails can now be moved within Thunderbird into the appropriate mail folders of the configured mailboxes. This is also a good opportunity to clean up emails or reorganize them into a new subfolder structure (for example archive folders “Inbox 2024‑2019” or “Sent 2024‑2020”, “Sent 2019‑2010”).
3.11) The end! Wishing all SUCCESS; and let yourself/your client enjoy a cleanly set up, completely fresh Thunderbird!
Further notes: When using antivirus programs that “break open” POP3‑SSL retrieval in order to scan incoming emails in email clients for infection risks or phishing links, it sporadically happens that a blockage occurs and the email program “connects endlessly into nirvana” during retrieval.
In such a constellation, a short‑term deactivation of the antivirus‑side email scanning for the duration of a single retrieval process provides a remedy.
For example, with G‑DATA: ** Thunderbird und G-DATA – Mail Abruf Zeitüberschreitung – was tun, wenn das eMail Abholen hängt oder klemmt
