What is MDA?
Also known as: Mail Delivery Agent
An MDA (Mail Delivery Agent) is a software component that transfers an email from a Mail Transfer Agent (MTA) into a recipient's local mailbox, typically on a mail server or a user's machine.
A Mail Delivery Agent (MDA) is the final hop in the email delivery chain. Its primary job is to accept a message from a Mail Transfer Agent (MTA) and deposit it into the correct user's mailbox storage. The MDA handles the actual writing of the message data to disk or to a mail store, and it is responsible for applying any final delivery rules, such as filtering spam, sorting into folders (via Sieve scripts), or invoking vacation auto-replies.
The MDA operates after the MTA has resolved the recipient's domain and determined that the message is destined for a local mailbox. Common MDAs include procmail, maildrop, and the built-in delivery agents inside MTAs like Postfix (local delivery) or Exim. Some MDAs also support the LMTP (Local Mail Transfer Protocol, RFC 2033), which is a simplified version of SMTP used for final delivery to a mail store. Unlike the MTA, which routes messages across the internet, the MDA never communicates with external servers; it works entirely within the local system or network.
In the broader email stack, the MDA sits between the MTA and the Mail User Agent (MUA). The MTA hands off the message to the MDA, which writes it into a mailbox format such as mbox, Maildir, or a database-backed store. The MUA (e.g., Outlook, Thunderbird) then reads the stored message via IMAP or POP3. The MDA is invisible to end users but is critical for ensuring that messages are reliably saved and organized on the server.
Key facts
- MDA receives mail from an MTA and writes it to a user's mailbox.
- Common MDAs include procmail, maildrop, and Dovecot's LDA.
- MDAs may apply Sieve (RFC 5228) filters for sorting and auto-replies.
- LMTP (RFC 2033) is often used between an MTA and MDA for final delivery.
- MDAs support mailbox formats like mbox, Maildir, and dbox.
How it works in practice
Related terms
References
More in Email
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Bounce Message
A bounce message, or non-delivery report (NDR), is an automated email sent to the originator when delivery cannot be completed, including a status code and failure reason.
Catch-All Email
A catch-all email account receives all messages sent to invalid or nonexistent addresses at a domain, preventing bounce-backs but attracting spam.
DKIM
DKIM is an email authentication method that lets a domain claim responsibility for a message by attaching a cryptographic signature validated against a DNS public key.
DMARC
DMARC is an email authentication policy protocol that builds on SPF and DKIM to tell receiving mail servers how to handle messages that fail authentication checks.
DNSBL
A DNSBL is a real-time IP reputation database published via DNS, used by mail servers at SMTP time to reject or flag senders known for spam or abuse.
Greylisting
Greylisting is a spam-filtering technique that temporarily rejects email from unknown senders, forcing legitimate mail servers to retry delivery while filtering out spammers that do not retry.
IMAP
IMAP is an email protocol that lets clients access and manage messages stored on a mail server, keeping the server as the authoritative copy and synchronizing folder state across multiple devices.
Mailbox Quota
A mailbox quota is a storage limit placed on a user's email account. Once the total size of stored messages exceeds this limit, the server may reject new incoming emails or warn the user.
MTA
An MTA (Mail Transfer Agent) is server software that routes email between domains using the SMTP protocol, acting as the core delivery engine for internet email.