I use Thunderbird via POP because I like to have a copy of all email on my computer so I can easily back it up.
I also use gmail with other clients (e.g. mobile phone) so I have the following server settings (accessible via Account Settings -> [account name] -> Server Settings):
- Check for new messages at startup.
- Check for new messages every 10 minutes.
- Automatically download new messages.
- Leave messages on server.
- Until I delete them.
Deleting emails
The ‘until I delete them’ option is interesting. If you delete an email in Thunderbird it’ll be moved to the Trash folder, but it will remain on the gmail server until it has been deleted from the Trash folder.