can I trash my mail folder
As a general rule: don't delete anything in ~/Library. Those are private folders used by apps to save configuration and data.
The ~/Library/Application Support/Mail folder in particular holds configuration files.
Don't delete ~/Library/Mail either, if your mail provider uses POP3 you'll lose all mail, if your mail provider uses IMAP the Mail app will have to download messages and attachments again.
To free up disk space delete mails you no longer need and then simply empty the trash: