Thunderbird is equipped with a junk email folder that spam and garbage mail goes to by default. The problem is, emails in the junk folder just sit around and take up space until you manually go in and delete them. If you’re careful about what you mark as junk, you won’t need to worry about this folder. Here’s how to send emails that you mark as junk directly to the trash.
Open Thunderbird and select Tools >> Options.
In the options window click the Security >> Junk tab. Then check both When I Mark Messages as Junk and Delete Them. Click OK to save changes.
Now emails that you mark as junk or spam will go straight to the trash and no longer take up space on your local disks.