how to clear mail cache

You are right: attachments that take up lots of space.

To get an idea if this is the case:

• Open Finder
• Click on Shift+cmd+G

1. On the Finder-pop-up-window you'll get, paste this command

~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads

• Click on Go
• Right-click on the folder Mail downloads
• Left-click on Get info

DO NOT delete downloads here: go to Mail, delete attachments there or archive these mails to an external drive

To find out how much space Mail thakes on your MBP, did you look at About this Mac/storage? This very often gives a wrong indication. Sometimes (but not always!!) it helps to rebuilt Spotlight index https://support.apple.com/en-us/HT201716 .In this link, only the folder documents is added, but you could add your entirely HDD.

if you want to know exactly how much space is left on your HD, go to Finder, right click on your HD and left-click on Get info.

Rebooting in safe mode can also clear caches. Be patient: this can take a long time (e.g. 10 to 15 minutes). https://support.apple.com/en-us/HT201262. Afterwards, reboot again in normal mode

Also, have a look at Finder, folder Downloads. At this Folder, often old downloads that you don’t need anymore can be found