MDA
also: Mail Delivery Agent
A Mail Delivery Agent (MDA) is a program that receives emails from a Mail Transfer Agent and stores them in a user's mailbox or applies filtering rules before delivery.
An MDA sits at the end of the email pipeline, handling the final step of message delivery. While an MTA (Mail Transfer Agent) like Sendmail or Postfix routes mail across networks, the MDA takes that mail and places it into the recipient's local mailbox or applies user-defined rules.
Common MDAs include procmail, maildrop, and dovecot's local delivery component. An MDA can filter spam, sort mail into folders, or execute scripts based on message headers. For example, procmail can read a .procmailrc file to automatically move emails from a mailing list into a specific folder.
In modern systems, the distinction between MTA and MDA has blurred—many MTAs like Postfix can perform both roles, directly delivering to local mailboxes without a separate MDA.