meet certain criteria, like subject or person.

In Mail, on the Menu Bar, choose Mailbox and then New Mailbox.
This creates what looks like a folder into which you can drag whatever mail you want.

Alternatively, you could create a Smart Mailbox and set its filter option to only display emails that meet your requirements. All emails from PayPal, for example.

The above works on any type of email system, POP3 or IMAP.

Apple runs an IMAP service,, and
These work particularly well on Apple Mail.

Google runs one too,