MailServer

A mailserver is a server that routes, stores and delivers electronic mail. In typical deployments, it comprises components for transport, mailbox storage, and user access. The core component is the mail transfer agent (MTA), which handles SMTP-based message submission and relaying. Mail delivery agents (MDA) deliver messages to mailbox storage, and mail submission agents (MSA) accept messages from users. Client applications (MUA) connect to the MSA or MDA via protocols such as SMTP, IMAP, or POP3. The mail server relies on DNS MX records to locate the recipient's server and may implement anti-spam and anti-virus filtering.

Messages are submitted from client to MSA via SMTP, possibly authenticated with SASL. The MTA routes the message to the recipient server, possibly across multiple MTAs. The recipient MTA stores the message in the recipient's mailbox via an MDA, from which the user retrieves it with IMAP or POP3. Modern servers often implement TLS encryption (STARTTLS) for SMTP, and TLS for IMAP/POP3, as well as SPF, DKIM, and DMARC to prevent spoofing.

Common software includes Postfix, Exim, and Sendmail as MTAs; Dovecot and Cyrus as IMAP/POP3 servers; and groupware solutions such as Microsoft Exchange or Zimbra. Deployments vary from on-premises to cloud-hosted services, and many organizations use a combination of internal servers and outsourced mail providers.