Is there any way to copy individual POP Mailboxes so I can store them on an external hard drive?

Have a recent restore failed to restore some of my POP Mailboxes (folders), I was wondering if there is a way to back them up or copy them to an external hard drive.

I am using Time Machine but to be reduntant I frequently copy documents to a thumb drive and/or other external drive.

I am not very computer literate so I cannot seem to figure out a way to do this.