I am running out of some disk space, and noticed Mail has about 30GB of emails stored.
What is the best way to delete these emails? I don't need them as a server has a copy.
Go to Mail->Preferences->Accounts
Choose the mail account
Aplles's docs does not say what happens when you chnage it but does have the comment
This is not an answer but an addendum to Mark's response (I'd put it in a comment but I don't have the reputation on AD yet to do so), since I came across this thread because I also wanted to get rid of Mail's cache.
I just want to confirm that the steps outlined by Mark (changing the status to "Don't keep...") does delete the retroactive temporary files that Mail.app used for your e-mails. You can confirm this by browsing to ~/Library/Mail/V2. Before I changed the status, this directory was just over 5 gigabytes. Afterward, it is now 37.2 MB. Also, since they are temp files, they do not go to the trash.
My small SSD thanks you!