I accidentally deleted all my mail on my MacBook Pro. Can I get it all back?

If you mistakenly delete messages, just drag them out of the Bin mailbox to another mailbox. Or select them in the Bin mailbox, then choose Message > Move to > [mailbox].

If you set up Time Machine to back up files on your Mac, you can use the backups to restore content that Mail downloaded (based on options set in Mail and on your Mac). See Restore items backed up with Time Machine.