I have no idea what cleaning means, but have you turned on the expunge on exit command in server settings.

Also, what version are you running? And what account details are shown at help > troubleshooting information?

I have no idea what cleaning means, but have you turned on the expunge on exit command in server settings.

Cleaning = Compact - wrong translation.

I've turned the expunge now and exit :) Nothing has changed - still 22Gb is the size of the folder...

Also, what version are you running? And what account details are shown at help > troubleshooting information?

This is my information:

 Application Basics

   Name: Thunderbird
   Version: 115.2.2
   Build ID: 20230912052425
   Distribution ID:

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:115.0) Gecko/20100101 Thunderbird/115.2.2
   OS: Windows_NT 10.0 19045
   OS Theme:

   Launcher Process: Enabled
   Multiprocess Windows: 0/0
   Fission Windows: 0/0
             Enabled by default
   Remote Processes: 3
   Enterprise Policies: Inactive
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false
   Memory Size (RAM): 31.8 GB
   Disk Space Available: 68.5 GB

 Mail and News Accounts
   account1:
     Libraries

     Library
     Status
     Expected minimum version
     Version in use
     Path

     RNP (OpenPGP)
     OK
     0.17.0
     0.17.0+PR2073.MZLA.115.2.2
     C:\Program Files (x86)\Mozilla Thunderbird\rnp.dll

 Remote Processes

   Type: Count

     Isolated Web Content: 1
     Extension: 1
     GPU: 1

 Add-ons

     Name
     Type
     Version
     Enabled
     ID

       British English Dictionary (Marco Pinto)
       dictionary
       3.3.0
       true
       marcoagpinto@mail.telepac.pt

       Polish Spellchecker Dictionary
       dictionary
       1.0.20160228.1webext

We need the detail that is in the section "Mail and News Accounts"

We need the detail that is in the section "Mail and News Accounts"

Wayne, here you are: ID Incoming server Outgoing servers Name Connection security Authentication method Name Connection security Authentication method Default?

account1 (imap) poczta23199.e-kei.pl:993 SSL/TLS Encrypted password poczta23199.e-kei.pl:465 SSL/TLS Normal password true

Mark, Wayne, any ideas what can I do with this issue?

Find your thunderbird profile, which you can determine in Help > Troubleshooting Info.

Since you are using imap, in Windows find the largest file - probably over 10gb. Shut down Thunderbird.

Tell us the file name and size.

If the file is in a subdirectory of imapmail you should be able to delete the. For safety move it to some directory outside the profile, documents or desktop. Start up Thunderbird. Is everything good?