Email – POP, IMAP, SMTP … What Does It All Mean?

by Billy J Long Guidance, PSA Articles

Article (PSA‑0017)

Why Knowing What Happens “Under the Hood” Helps

We all send and receive email every day, but only notice a problem when a message won’t go out or an inbox stays empty. Understanding the basic flow and the protocols involved can save you time, frustration, and even money.

Email Flow – From You to the Recipient

  1. You compose the message in an email client (Outlook, Thunderbird, Apple Mail, etc.) and click **Send**.
  2. The client talks to your outgoing mail server using the **SMTP** protocol (usually on port 587 or 465 with TLS). The server accepts the message and places it in a queue.
  3. The SMTP server looks up the recipient’s domain (e.g., example.com) via DNS MX records, then hands the message off to the recipient’s inbound server.
  4. The inbound server stores the message until the recipient’s client retrieves it.
  5. The recipient’s client uses either **IMAP** or **POP3** (both over TLS) to download the message, then displays it in the inbox.

Key Email Protocols

Outgoing – SMTP (Simple Mail Transfer Protocol)

  • Used **only** for sending mail.
  • Modern servers require encryption (STARTTLS on port 587 or SMTPS on port 465).
  • Often works with OAuth 2.0 authentication (e.g., Google, Microsoft 365) rather than plain passwords.

Incoming – IMAP vs. POP3

  • IMAP (Internet Message Access Protocol) – Port 993 (TLS)
    • Keeps mail on the server.
    • Syncs folders across all devices (phone, laptop, desktop).
    • Supports server‑side searching and multiple mailboxes.
  • POP3 (Post Office Protocol) – Port 995 (TLS)
    • Downloads mail to the local device and (by default) removes it from the server.
    • Good for a single device with limited storage, but makes multi‑device access painful.
  • Even when POP3 is used, most providers now keep a copy on the server for a short grace period.

Choosing the Right Receive Protocol

  • If you need to read mail on multiple devices (phone, tablet, work PC) – choose **IMAP**.
  • If you only ever use one device and want to store mail locally – POP3 will work, but IMAP is still the safer default.

Quick Troubleshooting Checklist

  1. Can you connect to the internet? Verify Wi‑Fi/Ethernet works.
  2. Sending problems?
    • Check SMTP server name, port, and encryption.
    • Confirm username/password (or OAuth token) is correct.
    • Look for any firewall or antivirus that might block outbound port 587/465.
  3. Receiving problems?
    • Verify IMAP (or POP3) server address, port, and TLS setting.
    • Make sure the account isn’t set to “offline” or “work offline”.
    • Check that your mailbox isn’t full (many providers impose a quota).
  4. Authentication errors? Many providers now require **app‑specific passwords** or **OAuth 2.0**; generate a new credential in your account portal.
  5. Still stuck? Capture the exact error message and give it to your IT support team – it often points directly to the mis‑configured setting.

Bottom Line

Understanding the three core protocols—SMTP for sending, IMAP/POP3 for receiving—lets you diagnose most common email issues quickly. Use IMAP whenever you want seamless access from multiple devices; stick with POP3 only if you have a strong need to keep mail solely on one machine.

Need a Hand?

If you have questions about configuring your email client, fixing sending/receiving problems, or setting up a more secure authentication method, call PSA Computer Services at (707) 506‑6802. We’ll get your inbox back on track.