Mailspring stores a cache of your email data on your computer, including all of the headers in your mailboxes and a limited set of email message bodies and attachments. Saving this information is required, and enables great client-side search and a snappy UI.
You can change where Mailspring's data is kept by creating a symlink on your system, redirecting the settings folder to another location or another drive.
1) First, quit Mailspring and identify where it is storing it's data:
$XDG_CONFIG_HOME/Mailspring or ~/.config/Mailspring