psmail

Psmail is a set of Perl scripts that, when used together, are similar to using an email program. Messages are stored one-per-file in specified directories, and utilities are given to create template new messages (writing headers, etc), reply, read, and list messages with relevant info (i.e. #lines, subject, who from, etc.). As each file is a separate message, you can also write complex commandlines to conditionally handle files. Psmail doesn't handle MIME or multiple POP accounts (yet).