What Is an IMAP Server? (And Why It Still Beats POP3 in 2026)

You’re setting up a new mailbox in Outlook. You send a test email from your phone, open your laptop, and the message is nowhere. Inbox looks different on every device. Folders don’t match. Sound familiar?

That sync problem is exactly what IMAP was built to solve. And despite being around for decades, IMAP is still the protocol running behind almost every modern email client.

In this post, I’ll break down how IMAP servers actually work, how IMAP compares to POP3, which ports and security settings matter, and why all of this is relevant if you’re managing multiple mailboxes for outbound.

How an IMAP server works

Think of IMAP like a shared Google Doc. Everyone sees the same version, from any device, at any time. POP3, by contrast, is more like downloading a file to your desktop: once it’s on your machine, the original is gone.

That’s the core idea behind IMAP (Internet Message Access Protocol). IMAP allows a client to access and manipulate electronic mail messages on a server. Your email stays on the server. Your devices pull a synchronized view of it.

Here’s the flow:

1. 1.

   You send an email. The first protocol in action is SMTP (Simple Mail Transfer Protocol). SMTP moves the email from the sender to the sending server, then to the receiving IMAP server.

2. 2.

   The recipient connects to the IMAP server through their email client. The email client displays headers first, then downloads full message bodies on demand.

3. 3.

   Any action the recipient takes (reading, deleting, moving to a folder) syncs back to the server. Every other connected device sees the change.

The current IMAP base specification is RFC 9051 (IMAP4rev2), which obsoletes RFC 3501 (IMAP4rev1). You’ll still see “IMAP4rev1” terminology referenced in most real-world servers and documentation, since RFC 3501 was the long-standing base IMAP specification from 2003, and it’s still widely implemented and referenced.

Typically, you need to add your email provider’s IMAP server address to your email client. If you’re using Gmail inside Gmail’s own interface, that’s already handled. But if you want to connect a Gmail address to Thunderbird, Apple Mail, or another client, you’ll enter the IMAP server details manually.

Common IMAP server settings

No competing glossary page gives you the actual server addresses and ports in one place. Here they are:

For Gmail, Outlook, Yahoo, and iCloud, use port 993 with SSL/TLS, and OAuth 2.0 or an app password for authentication (basic auth was deprecated in 2022). Microsoft is ending support for Basic Authentication for IMAP in December 2026, so if you’re still using plain username/password login for Outlook or Microsoft 365, now is the time to switch.

IMAP vs. POP3

This is the comparison most people are actually looking for when they search “what is an IMAP server.” Both protocols retrieve email, but they work very differently.

IMAP synchronizes emails across all your devices by keeping messages on the server, as opposed to POP3 which downloads and deletes them.

One concession: POP3 still has a use case. If you’re on a tight mailbox quota and only check email from one machine, POP3 lets you pull everything down and free up server space. POP3 has its uses, mainly for people managing a very full mailbox who want to free up server storage, but those are edge cases. For everyone else, especially anyone managing multiple mailboxes or sending outbound, IMAP is the right call.

Advantages of using an IMAP server

IMAP has clear benefits over POP3 and older retrieval methods. Here are the ones that matter most:

• •

  Your inbox stays in sync across every device and email client. Read a message on your phone, and it’s marked read on your laptop. Delete it in Thunderbird, and it’s gone in webmail too.

• •

  IMAP initially downloads only headers, saving bandwidth and storage. Full message bodies and attachments load only when you open them.

• •

  You can create, rename, and delete folders directly on the server. The folder structure syncs to every connected client.

• •

  IMAP also provides the capability for an offline client to resynchronize with the server. So if you lose connection, your email client queues changes and pushes them once you’re back online.

Drawbacks to watch for

IMAP isn’t perfect. Here are the real tradeoffs:

• •

  You depend on the server being available. If your provider has an outage or the server is unstable, you lose access until it’s back, even though cached messages may still be visible in your client.

• •

  Every message stored on the server counts against your provider’s storage quota. Exceed it and you’ll hit issues sending and receiving.

• •

  The original protocol on port 143 has a critical vulnerability: it transmits all data in plaintext. That includes your login credentials and message content. Always use port 993 with TLS encryption.

• •

  Managing many IMAP-connected mailboxes at once (common in sales teams running outbound) can create sync conflicts or throttling, depending on the provider.

IMAP security: port 143 vs. port 993

This deserves its own section, because “security vulnerabilities” is too vague to be useful.

IMAP uses port 143 for unencrypted connections and port 993 for IMAP over TLS/SSL (IMAPS), as defined by the IANA port registry.

Plain IMAP on port 143 sends the username, password, and messages in cleartext. Anyone intercepting the traffic (say, on a shared Wi-Fi network) can read everything. Port 993 is the secure alternative. When a client connects to port 993, a TLS tunnel is negotiated before any IMAP commands are sent.

In practical terms: in modern networking, unencrypted port 143 is considered obsolete for public internet connections. Every major provider (Gmail, Outlook, Yahoo, iCloud) defaults to port 993 with SSL/TLS. If you’re connecting a mailbox for any purpose, confirm that encryption is active.

This matters especially for teams managing multiple sending mailboxes for cold outreach. Each mailbox connection is an attack surface. If one mailbox is misconfigured on an unencrypted port, credentials can leak, the mailbox gets compromised, and your sender reputation takes the hit. Keeping every connected mailbox on port 993 with proper authentication is a baseline for inbox health.

And inbox health isn’t just about security. It’s about deliverability: warm-up, reputation monitoring, and spam placement tracking across every mailbox you operate. That’s where a tool like lemwarm fits in, keeping your sending mailboxes healthy so they actually land in the inbox instead of spam.

Final thoughts

IMAP isn’t going anywhere. Even though email technology keeps evolving fast, the core problem IMAP solves (syncing a single mailbox view across many devices) is more relevant now than it was ten years ago. We all check email on phones, laptops, tablets, and webmail. IMAP makes that work.

If you’re on a sales team running outbound from multiple mailboxes, understanding IMAP basics isn’t optional. It’s the protocol underpinning your mailbox connections, your folder sync, and ultimately your deliverability.

For a deeper dive into the protocol itself, we have a companion post on what IMAP is.

And if you’re ready to make sure every one of those mailboxes is warmed up, monitored, and landing in the inbox, start a 14-day free trial of lemlist and see for yourself.