From resources.infosecinstitute.com
The Simple Mail Transfer Protocol (SMTP) is one of three protocols used for email. The other two are Post Office Protocol v3 (POP3) and Internet Message Access Protocol (IMAP).
SMTP is one of several internet protocols that are designed to be plaintext and ASCII printable. This means that traffic sent over SMTP is visible and easily readable by eavesdroppers. When running in plaintext mode, SMTP uses port 25. Port 587 is the official port that should be used by SMTP clients submitting traffic to be routed by a mail server.