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
- Check the user name is the full address.
- Check the host, then the port and the security choice together; a mismatch, such as STARTTLS on port 993, fails.
- Check that the domain is active in the client area; a domain waiting for its proof token does not yet accept mail.
- Try the webmail with the same password. If it works there, the password is right and the client’s settings are the problem.
A quick test before you give up
Send a message to an address at another provider, then reply to it. If sending works and receiving does not, the problem is in the incoming settings, and the reverse points to outgoing. Trying one direction at a time saves changing everything at once.
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.
Is port 587 for IMAP?
No. Port 587 is SMTP submission, used for sending. IMAP reads mail on port 993 or 143, so a program that asks for an IMAP port should not be given 587.
Which port does the outgoing mail server use?
Outgoing mail is SMTP, not IMAP: port 465 with SSL/TLS, or port 587 with STARTTLS. IMAP has no outgoing port of its own.
Which ports does POP3 use?
Port 995 with SSL/TLS, or port 110 with STARTTLS. Prefer 995.
Sources
The documents this guide’s statements rest on. Each link opens in a new page.