Automatically save email as html/text/rtf in a local folder?

i would like to setup an email client to have a filter such that whenever something happens (eg, an email is sent from a particular person), the email is automatically saved to some folder (that i specify), in some generic format (eg, txt, html, etc.). does anybody know of a good way to set this up? i couldn’t figure it out on mail or thunderbird (i’m on a mac). thanks