Mail Server
A mail server is a computer system or application dedicated to sending, receiving, and storing email messages, forming the infrastructure backbone that makes email communication possible. Mail servers typically comprise two components: the outgoing mail server (Mail Transfer Agent or MTA), which sends emails from one server to another using the SMTP protocol, and the incoming mail server, which stores and delivers messages to recipients’ mailboxes using either IMAP or POP3 protocols.
Mail servers apply filtering rules at the point of receipt, evaluating incoming messages for authentication compliance, spam signals, and blacklist status before deciding whether to deliver them to the inbox, route them to spam, or reject them outright. This is where SPF, DKIM, and DMARC authentication is verified: the receiving mail server checks the authentication records in your domain’s DNS and applies the sender’s DMARC policy to any messages that fail the checks.
For B2B email marketers, mail server infrastructure is largely abstracted away by email service providers, but understanding its role helps demystify deliverability. The quality of your ESP’s sending infrastructure, including the reputation of their mail server IPs, their compliance practices, and their relationships with major inbox providers, directly affects your email’s ability to reach the inbox. When evaluating ESPs, asking about their mail server architecture and deliverability support is a reasonable due diligence question.
What is the difference between an SMTP server and an IMAP server?
SMTP (Simple Mail Transfer Protocol) is the protocol used to send emails from one server to another, including from your email client or marketing platform to the recipient’s mail server. It is the sending protocol. IMAP (Internet Message Access Protocol) is a protocol used by email clients to retrieve and synchronise messages from a mail server to the user’s device, allowing emails to be accessed from multiple devices while remaining stored on the server. SMTP is for sending; IMAP and POP3 are for receiving.
What is a Mail Transfer Agent (MTA)?
A Mail Transfer Agent (MTA) is software that routes and delivers email between mail servers using the SMTP protocol. When your email platform sends a campaign, its MTA handles the actual transmission of each message from the platform’s infrastructure to the receiving mail servers. The MTA manages delivery queues, handles retry logic for temporarily failed deliveries, processes bounce notifications, and communicates with spam filtering systems during the sending process. Commercial email marketing platforms manage their own MTA infrastructure on behalf of their customers.
How does a mail server affect email deliverability?
The mail server’s role in deliverability is twofold. On the sending side, the IP reputation of the sending mail server is one of the primary signals that receiving servers use to decide whether to accept, filter, or reject an email. On the receiving side, the receiving mail server applies all filtering logic, checking authentication records and spam signals, and ultimately deciding where to place the incoming message. Your ability to influence the receiving server’s decision is through the quality of your sending practices: list hygiene, authentication, and engagement rates.