What is a mail server?

Mail server

A mail server is a computer system that sends, receives, and stores email. It acts as the digital post office for your messages, routing them from sender to recipient across the internet using standardized protocols like SMTP, IMAP, and POP3.

When you hit send on an email, your message doesn't travel directly to the recipient. It passes through mail servers that authenticate, process, and deliver it to the right inbox. Understanding how this works helps you troubleshoot email deliverability issues and keep your messages out of spam folders.

How mail servers work

Mail servers handle two distinct jobs: sending outgoing mail and receiving incoming mail. Different server types manage each task.

Outgoing mail servers (SMTP) accept messages from your email client, verify your identity, and route the message toward its destination. SMTP stands for Simple Mail Transfer Protocol, the standard language servers use to communicate.

Incoming mail servers (IMAP/POP3) store messages until you're ready to read them. IMAP keeps emails on the server so you can access them from multiple devices. POP3 downloads messages to a single device and typically removes them from the server.

Here's the journey your email takes:

1. You compose and send a message
2. Your email client connects to an outgoing SMTP server
3. The SMTP server checks DNS records to find the recipient's mail server
4. The message transfers to the recipient's incoming mail server
5. The recipient's email client retrieves the message

Each step involves authentication checks that help prevent spam and verify sender identity.

Why mail servers matter for deliverability

Your choice of mail server infrastructure directly affects whether your emails reach the inbox or land in spam. Mail servers handle critical authentication protocols like SPF, DKIM, and DMARC that prove you're a legitimate sender.

When these protocols aren't configured correctly, receiving servers get suspicious. They might reject your message outright or flag it as potential spam. You can verify your authentication setup using a DKIM checker to catch configuration issues before they hurt your inbox placement.

Mail servers also track your sending reputation. If your server's IP address has been used to send spam, other servers may block or filter messages from that address. This is why shared hosting email often struggles with deliverability, and why dedicated email infrastructure matters for business communication.

Types of mail servers

Shared mail servers handle email for multiple users or organizations on the same infrastructure. They're affordable but come with risk: if another user on the server sends spam, your reputation suffers too.

Dedicated mail servers give you exclusive use of the server and its IP addresses. You control your own reputation, but you're also responsible for maintaining it.

Cloud-based email services like ActiveCampaign manage the server infrastructure for you. They maintain relationships with major inbox providers, monitor domain reputation, and handle the technical complexity of authentication protocols.

Common mail server problems

Bounced emails happen when a mail server can't deliver your message. Hard bounces mean the address doesn't exist, while soft bounces indicate temporary issues like a full inbox. Learn more about handling bounced emails to protect your sender reputation.

Blacklisting occurs when your server's IP address gets flagged for suspicious activity. Once blacklisted, your emails may be blocked entirely by receiving servers. Following best practices to avoid email blacklists keeps your messages flowing.

Authentication failures happen when SPF, DKIM, or DMARC records are missing or misconfigured. These failures tell receiving servers they can't verify you're who you claim to be.

FAQs

What's the difference between SMTP and IMAP?
SMTP handles sending email, while IMAP handles receiving and storing it. Your email client uses both: SMTP when you send a message, IMAP when you check your inbox.

Do I need my own mail server?
Most businesses don't. Email service providers handle server management, deliverability optimization, and authentication setup. Running your own server requires significant technical expertise and ongoing maintenance.

Why do my emails go to spam even with a working mail server?
Server function is just one factor. Email engagement also matters: if recipients don't open or interact with your messages, inbox providers may start filtering them as unwanted.

Ready to improve your email deliverability without managing server infrastructure? Start your free ActiveCampaign trial and let us handle the technical details.