Dear Łukasz, Happy new year. That is possible with archiving: https://support.mozilla.org/en-US/kb/archived-messages There's also an add-on that does it automatically, search for "AutoarchiveReloaded" in Tools > Add-Ons In case you're using POP3 (see Account Settings > Server Settings) there are also options for that, just browse through the settings. Probably your server is using IMAP, in that case on your computer you see the same content as on the server. Then archiving comes in handy. Or manually dragging them into local folders once in a while. cheers

create corresponding sub-folders under "Local Folders" for those mail-account(s) in TB , which you are trying "save storage in my email box" , so under "Local Folders" create sub-folders like these: mail-account-name-01-Inbox mail-account-name-01-Sent ... mail-account-name-02-Inbox mail-account-name-02-Sent ...

in TB goto the mail-account that you want to reduce remote-mail-storage size.

change email SORT order : date wise ascending or descending. select OLD emails. learn how to use Ctrl/Command , Shift , Alt/option , etc buttons to select multiple emails. then select MOVE option & also select their destination corresponding sub-folders under the "Local Folders".

use TB, to do email moving operations.

do not loose this TB-profile folder , as it has LAST copy of your emails. all TB-profile sub-folder(s) are here, which has all emails,settings,etc,etc: in Windows: "%APPDATA%\Thunderbird\Profiles\" thats usually here: "C:\Users\<WindowsUserName>\AppData\Roaming\Thunderbird\Profiles\" in macOS: "~/Library/Thunderbird/Profiles/" thats usually in default mac drive Volume : "/Users/<MacOsUserName>/Library/Thunderbird/Profiles/" in Linux/Unix: "~/.thunderbird/Profiles/" thats usually in default volume : "/Users/<LinuxOrUnixUserName>/.thunderbird/Profiles/"

you can also change option inside TB , to auto move certain emails based on certain rules into their corresponding sub-folder under the "Local Folders" , but IMHO thats not necessary , if you move only OLD emails MANUALLY by yourself into "Local Folders" , after some time periodically , let say, after every 3 months or so.

you can decide what you want to move/backup/archive inside the "Local Folders", & that is better in most cases.

emails which are NEWER , should not be moved into "Local Folders" , unless they have attachments that are very big file size , in such case(s) you can create "MESSAGE FILTER" rule inside TB , to move only those emails into "Local Folders".

also see this backup/archiving tool.