Take a look in Mail
Your email application is full of stuff. All those Mail attachments mount up over the years, and while you need to keep some of them some of the time, you probably don’t need to retain all of them forever. The best practice is to delete attachments in emails you no longer need; you can do this by deleting the message itself or selecting a message and choosing Remove Attachments in the Messages menu.
You can also create a search in Mail that lets you identify emails containing attachments. Try Mailbox>New Smart Mailbox, select “contains attachments” and save. This is a very unsophisticated tool that just makes it easier for you to monitor any emails you might have received that contain attachments, though it still makes for a very manual process. This is actually the problem with Mail: it doesn’t let you easily manage emails containing large attachments. It does let you do one more thing, however, which you should do now: Open Mailbox and choose Erase Junk Mail to get rid of all the junk that has accumulated. You should also select Erase Deleted Items.
Run Onyx or CleanMyMac
There are numerous applications that claim to help you free up and better manage space on your Mac. I like the free Onyx application, which has been my go-to troubleshooting solution for years. But many users also like MacPaw’s CleanMyMac application. What these applications do is make it possible to delete data you can’t easily or safely get to on your Mac, including unwanted database files, bloated logs, and more. Apple says that macOS will automatically clear such data — including temporary database files, interrupted downloads, staged macOS and app updates, Safari website data, and more — when space is needed on your Mac. But some users might prefer to be proactive.
