Remote Access and Mail Protocols

Beyond the web, technicians rely on protocols for remote management and email. This lesson covers SSH, Telnet, RDP, and the mail trio of SMTP, POP3, and IMAP.

Each has a standard port and a security profile you should commit to memory for the exam.

SSH (Secure Shell) provides an encrypted command-line connection to a remote device. It uses TCP port 22.

SSH is the standard for securely managing servers, switches, and routers. All commands and credentials are encrypted, making it safe to use even across untrusted networks.

Telnet offers a remote command line like SSH but sends everything in plain text, using TCP port 23.

Because passwords and commands travel unencrypted, Telnet is insecure and largely obsolete. It survives only on isolated legacy systems; always prefer SSH for remote access.

RDP (Remote Desktop Protocol) gives a full graphical desktop session on a remote Windows machine. It uses TCP port 3389.

RDP is widely used for remote administration and support. Because it exposes a desktop, it is a common attack target, so it should be protected behind a VPN or restricted firewall rules.

SMTP (Simple Mail Transfer Protocol) sends email and relays it between mail servers. It uses TCP port 25.

Think of SMTP as the outgoing mail protocol. Secure submission of mail by clients often uses port 587, while 465 is used for implicit TLS, but 25 is the classic server-to-server port.

POP3 (Post Office Protocol version 3) retrieves email, typically downloading messages to one device and removing them from the server. It uses TCP port 110.

POP3 suits a single-device setup. Its secure version, POP3S, uses port 995. Because it usually deletes server copies, it does not sync well across multiple devices.

IMAP (Internet Message Access Protocol) also retrieves email but keeps messages on the server and syncs across devices. It uses TCP port 143.

IMAP is ideal when you read mail on a phone, laptop, and desktop, since the server holds the master copy. Its secure version, IMAPS, uses port 993.

Keep the mail roles straight:

• SMTP sends and relays mail (outgoing).
• POP3 and IMAP retrieve mail (incoming).
• POP3 typically downloads and deletes; IMAP keeps and syncs.

A complete email flow uses SMTP to send and POP3 or IMAP to read.

Most mail protocols have encrypted versions on their own ports:

• SMTP submission: 587 (or 465 implicit TLS)
• POP3S: 995
• IMAPS: 993

The exam may ask for either the plain or secure port, so know both. Modern mail clients use the encrypted variants by default.

Lock in the remote access ports:

• SSH: TCP 22 (secure)
• Telnet: TCP 23 (insecure)
• RDP: TCP 3389 (graphical)

Always favor encrypted protocols: SSH over Telnet, and protect RDP carefully because it exposes a full desktop.

You can now recognize the everyday remote access and mail protocols with their ports and security traits. Combined with the web and file protocols, you have a solid map of common services.

This protocol knowledge underpins firewall rules, troubleshooting, and the DNS lessons that follow.

Pick the right protocol.

You learned remote access and mail protocols.

• SSH = TCP 22 (encrypted shell); Telnet = TCP 23 (plain text, obsolete).
• RDP = TCP 3389, graphical remote desktop.
• SMTP = TCP 25 sends mail; POP3 = 110 and IMAP = 143 retrieve it.
• POP3 downloads/deletes; IMAP keeps and syncs across devices.
• Secure ports: SMTP 587, POP3S 995, IMAPS 993.