Mail is taking up too much storage on MacBook! How do I save more storage?

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 significant 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 from happening in the future? Many thanks in advance!