Mail is taking up too much storage on MacBook!

I have several email accounts that are linked to my Mail App on my MacBook Pro.

This has been on there for several years and it's taking up a huge space on my hard drive.

I've found the MAIL file and have backed this up. How can I reduce the size of this folder on my hard drive? Is there a way of deleting the folders on there or must I delete the emails on the Mail App.

Also if I delete the emails from places like gmail and hotmail on my Mail App does this delete them permanently or are they still accessible if I access them through safari?

Also any advice on how to stop this happening in the future?

Many thanks in advance!