Support | Mail Settings
Archiving online mail folders
There is a limit to online mail space and therefore you may need to Archive messages in your online folder.
This is fairly easy to do and means you have local access to older mails and attachments.
You can create a local folder, all methods move emails out of your main mailbox to save space while keeping them searchable
For Thunderbird you will be able to create a folder in your mailbox alongside your existing email accounts. You can then create a filter to then automatically move messages older than 6 mon ths to the archive folder.