Mailservers

Mailservers are computer systems that handle the transmission, delivery, storage, and receipt of email for a domain or organization. They typically comprise a Mail Transfer Agent (MTA) that routes mail between servers, a Mail Delivery Agent (MDA) that places messages into mailboxes, and a Mail Submission Agent (MSA) used by clients to send mail. Mail user agents (MUAs) read and compose messages. Core protocols include SMTP for sending and IMAP or POP3 for retrieval, with DNS MX records guiding routing. SPF, DKIM, and DMARC support authentication and deliverability.

When a user sends mail, the client submits via SMTP to the MSA or MTA, which relays the message to the recipient's server. The receiving MTA hands the message to an MDA for storage, from which the MUA retrieves it using IMAP or POP3. Encryption is commonly provided with TLS or STARTTLS to protect transit.

Security and policy features are central. SASL-based authentication controls access; transport encryption with TLS protects messages in transit. Domain-level policies such as SPF, DKIM, and DMARC help verify sender identity and reduce spoofing. Spam filtering, antivirus scanning, and rate limiting are widely used. Deployments may include redundancy, load balancing, and monitoring to maintain reliability and availability.

Deployment models range from on‑premises servers to cloud-based services. Common software stacks combine MTAs such as Postfix, Exim or Sendmail with MDA/IMAP components (for example Dovecot) and, in some ecosystems, full solutions like Microsoft Exchange. Choice depends on scale, integration needs, and management preferences.

Operational considerations include uptime, backups, and compliance. Deliverability relies on good authentication practices and maintaining a clean IP reputation. Misconfigurations or open relays can lead to abuse and delivery failures.