The term you are looking for is Mail Delivery Agent, though the part about using a simple directory-based mail spool is usually more a factor of the Mail User Agent - you would use the format your email reader (the MUA) can read.
The aforementioned procmail is probably the one you would want to use. The spool format that is closest to what you seem to want would probably be maildir, though mbox might suit as well.
You will probably want a local mail reader such as Thunderbird,
Balsa or Mutt as well, unless the idea is that the mail is being dropped off for some program you are writing.
Prior to the mid-1990s, MDA functions would usually have been handled by a Mail Transfer Agent such as Sendmail or Postfix, as most users would have had timesharing accounts on the same system as the mail server, while those who didn't would usually have hosted their own mail servers locally. After the start of The September That Never Ended, however, most users were picking up their mail remotely using a POP3 server on their ISPs system, or later, on a dedicated webmail host such as Hotmail or GMail.
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