Save Space in the OS X Mail App by Disabling Attachment Downloads
The OS X Mail app can take up a lot of space on your computer as it downloads every email. If you’re short on space, How-To Geek suggests turning off the “Automatically download all attachments” option.
All you have to do is open Mail, then go to Mail> Settings> Accounts> Advanced. Uncheck “Automatically download all attachments” and you’ll start saving some space. Otherwise, you can also contact your email provider and tinker with some of the settings to save space. See How-To Geek for details.
How to Make Your Mac’s Mail Application Not Waste Gigabytes of Space | How-To Geek