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A message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox.[1] It is also called a local delivery agent (LDA).
Within the Internet mail architecture, local message delivery is achieved through a process of handling messages from the message transfer agent, and storing mail into the recipient's environment (typically a mailbox).
Spam filtering usually occurs at the MDA under modern email architectures.[2]
Many mail handling software products bundle multiple message delivery agents with the message transfer agent component, providing for site customization of the specifics of mail delivery to a user.
On Unix-like systems, procmail and maildrop are the most popular MDAs. The Local Mail Transfer Protocol (LMTP) is a protocol that is frequently implemented by network-aware MDAs.[citation needed]
The mail delivery agent is generally not started from the command line, but is usually invoked by mail delivery subsystems, such as a mail transport agent, or a mail retrieval agent.
All my experience says that the right place to do mail filtering is the MDA. It has all of the information and has all the options about what to do with the mail. There are MUAs that try to do filtering but that's mostly an historical artifact from a long time ago when MDA filtering didn't happen.
