mailtransfer

Mail transfer is the process by which electronic mail is moved from a sender’s mailbox to a recipient’s mailbox across computer networks. It is mediated by mail transfer agents (MTAs) that relay messages between servers, typically using the Simple Mail Transfer Protocol (SMTP) for inter-server transfer. Mail transfer is distinct from local delivery, which places the message into the recipient’s mailbox on the final server and is usually performed by a mail delivery agent (MDA).

Overview of operation: a user composes a message in a mail user agent (MUA) and submits it to an MSA or directly to an MTA. The sending MTA queries DNS for the recipient’s domain MX records to locate the destination MTA, then transfers the message. The destination MTA queues the message for delivery or hands it to an MDA for local delivery if appropriate. If delivery fails, the source MTA retries according to its queuing policy and may generate a bounce.

Security and authentication: transport between MTAs is commonly encrypted with TLS and supports STARTTLS to upgrade connections. Client submission usually requires authentication. Anti-spam and anti-abuse measures accompany mail transfer, including SPF, DKIM, and DMARC checks, reputation systems, and content filtering.

Implementation and terminology: Popular MTAs include Postfix, Exim, Sendmail, qmail, and commercial solutions such as Microsoft Exchange. The mail transfer process may use LMTP for direct delivery to a mailbox server, and inter-server delivery typically uses SMTP. DNS configuration and MX preferences influence routing and reliability; queue management affects delivery timing and failure handling.

By coordinating these components, mail transfer enables delivery from senders to recipients across the Internet, subject to domain policies and network conditions.