It's good to have them all on the IMAP email server, of course, in backup copies at the email service and locally in an email program. Is is really necessary for Mozilla Thunderbird, which you use now and then for a particular purpose, to feverishly start downloading all new mail when you start it? Must it store giga- upon gigabyte of old mail, too?
Whether you use Mozilla Thunderbird only sporadically or just want to preserve disk space on a mobile machine, you can set it up to store only the most recent messages on your computer, and what counts as recency is mostly up to you.
Make Mozilla Thunderbird Store Less Mail Locally for IMAP Accounts
To set up Mozilla Thunderbird to keep only a certain amount of mail locally (for fast search) in an IMAP account:
- Select Tools | Account Settings… from the menu in Mozilla Thunderbird.
- Go to the Synchronization & Storage category for the desired account.
- Make sure Synchronize the most recent is selected under Disk Space.
- Choose the time for which you want Mozilla Thunderbird to keep a local copy of mail.
- Select 18 Months, for instance, to have one and a half years' worth of email available offline and for speedy search.
- Click OK.
Older messages will still appear in the IMAP account's folders. It is just the message text that is not kept on your computer for faster access. If you delete such an older message, it will be deleted on the IMAP server, too.
To search all mail — including mail only available in full on the server —, select Edit | Find | Search Messages… from the menu and make sure Run search on server is checked.
(Updated October 2011)