my classic is an mbox filter. Read a mailbox, determine who sent it and when, and rewrite an output mailbox with different filter conditions applied (e.g., only

my classic is an mbox filter. Read a mailbox, determine who sent it and when, and rewrite an output mailbox with different filter conditions applied (e.g., only mails from nigerian princes, only mails younger than a month). determines for me how file I/O, error handling, indices / data structures pointing to messages work, sorting, etc.