MacMail taking over my hard drive

Can anyone help please?
I use MacMail to keep multiple emails in one place.
The gmail accounts (there are three of them) are taking over my hard drive.
One of them is apparently taking 44GB of my hard drive right now. When I checked, the Mail folders are full of .emlxpart files.
Can anyone tell me what these are and why there's taking so much space?
Can I just delete them or will they just be downloaded from somewhere again?
Any help would be v appreciated.
Thanks.

You can try to delete them but they may come back, I would try rebuilding the mailboxes.

Thank you. How do you rebuild them?