Guide · Setting up

IMAP, SMTP and POP3 settings: which ports and security to use

A mail program needs a host, a port and a security setting for reading and for sending. This guide says what each choice means and which to prefer.

Updated

What a mail program asks for

Reading mail uses IMAP, which keeps messages on the server and shows the same mailbox on every device, or POP3, which downloads them. Sending uses SMTP. For each, the program asks for the host, a port and a connection security: SSL/TLS, or STARTTLS.

The home page of the service lists the host and the ports it offers. The user name is the full address, and the password is the mailbox’s own.

The ports and what they mean

  • IMAP, port 993, SSL/TLS. The encrypted connection starts at once. This is the usual choice.
  • IMAP, port 143, STARTTLS. The connection starts in the clear and is upgraded to encryption.
  • SMTP, port 465, SSL/TLS. Encrypted from the first byte, for sending.
  • SMTP, port 587, STARTTLS. The submission port, upgraded to encryption.
  • POP3, port 995 (SSL/TLS) or 110 (STARTTLS). Only when a program cannot use IMAP.

Why encrypted from the start is preferred

RFC 8314 recommends that mail user agents and providers discourage cleartext protocols for mail access and submission, and prefer a TLS handshake that begins immediately on the ports 993 (IMAP), 995 (POP3) and 465 (submission). Where a program offers both, choose SSL/TLS on those ports; STARTTLS is for programs that cannot.

If it will not connect

  1. Check the user name is the full address.
  2. Check the host, then the port and the security choice together; a mismatch, such as STARTTLS on port 993, fails.
  3. Check that the domain is active in the client area; a domain waiting for its proof token does not yet accept mail.
  4. Try the webmail with the same password. If it works there, the password is right and the client’s settings are the problem.

Questions

Which port should be used for IMAP?

Port 993 with SSL/TLS, which encrypts the connection from the start.

What is the difference between SSL/TLS and STARTTLS?

SSL/TLS encrypts the connection from the first byte. STARTTLS begins in the clear and upgrades to encryption, so it depends on that upgrade succeeding.

Is POP3 needed?

Only for a program that cannot use IMAP. IMAP keeps the mailbox the same on every device.

Sources

The documents this guide’s statements rest on. Each link opens in a new page.

  1. RFC 8314: Cleartext Considered Obsolete: Use of TLS for Email Submission and Access