OWASP API Security Top 10 - 1

Telnet

A Telnet server uses the Telnet protocol to listen for incoming connections on port 23.

Could be spied on by wireshark because the the communication is not secure

HTTP

Works on port 80 or 8080 and sends data as plaintext

FTP

Transfer files on port 21

SMTP

simple mail transfer protocol on port 25

Email delivery over the Internet requires the following components:

1. Mail Submission Agent (MSA)
2. Mail Transfer Agent (MTA)
3. Mail Delivery Agent (MDA)
4. Mail User Agent (MUA)

POP3

Post Office Protocol version 3 used to download email from MDA on port 110

IMAP

Internet Message Access Protocol (IMAP) is more sophisticated than POP3. IMAP makes it possible to keep your email synchronized across multiple devices (and mail clients) on port 143