Mail Server (Mail Exchange Server)

What Is a Mail Server?

A mail server is a computer system (or cloud service) that handles the sending, receiving, routing, and storage of email. Mail servers use standardized protocols — primarily SMTP for sending, and IMAP or POP3 for retrieval — to communicate with each other and with email clients. When you send an email, it passes through at least two mail servers: your outgoing mail server (SMTP) and the recipient's incoming mail server (the one that stores the message in their mailbox).

Types of Mail Servers

MTA (Mail Transfer Agent): handles SMTP sending and routing between servers. Examples: Postfix, Exim, Sendmail, Microsoft Exchange Server. MDA (Mail Delivery Agent): delivers mail from the MTA to individual mailboxes. Examples: Procmail, Dovecot. MRA (Mail Retrieval Agent): handles IMAP and POP3 access for email clients. Often combined with the MDA. In practice, many commercial mail services (Google Workspace, Microsoft 365) combine all these functions into a single unified platform.

MX Records and Mail Server Discovery

When an email is sent to [email protected], the sending server looks up the MX (Mail Exchange) records in DNS for company.com. These records specify which mail servers are responsible for receiving email at that domain, ranked by priority. The sending server connects to the highest-priority MX server and initiates an SMTP conversation. If that server is unavailable, it tries the next lower-priority server. This redundancy ensures delivery even when one server is down.

Mail Server Authentication and Security

Modern mail servers must be configured with proper authentication to maintain deliverability: SPF records authorize which IPs can send as your domain, DKIM keys sign outgoing messages, DMARC policies define handling of authentication failures, and TLS encryption protects messages in transit. Servers without these configurations are treated as suspicious by receiving servers. Running a mail server without proper authentication is increasingly like showing up to an event without an ID — you'll be turned away.

Mail Servers and Email Verification

Email verification relies on mail server behavior during the SMTP handshake. EmailVerify.io connects to the recipient domain's mail server and queries whether a specific mailbox exists — without sending an actual message. The mail server's response (accept, reject, or defer) tells the verifier whether the address is valid. Catch-all servers complicate this by accepting all queries regardless of mailbox existence. Verifiers must account for this behavior to avoid false positives. Learn more at emailverify.io.

Choosing a Mail Server or Email Service

Most organizations today use a hosted email service rather than running their own mail server. Google Workspace and Microsoft 365 are the dominant options for business email. For high-volume transactional email, dedicated ESPs like SendGrid, Postmark, or Mailgun provide optimized sending infrastructure with built-in deliverability tooling. Running your own mail server is technically possible but requires significant expertise to maintain authentication, reputation, and security correctly.