Simple Mail Transfer Protocol is the protocol used to send email on the Internet. It is used to send mail between mail servers, and is also used to send mail from an email client to an outgoing mail server.
Sending mail between servers
For this purpose, SMTP is typically operated on TCP port 25. To prevent spoofing an email address, most common servers check the sender against a Sender Policy Framework DNS record, in which the owner of a domain specifies which servers can send mail from that domain. There is generally no other form of authentication used.
Sending mail to an outgoing server
SMTP is also used to send mail from a desktop email client such as outlook to the server designated by your email service. For this purpose, the connection is frequently encrypted, and frequently requires authentication in order to prevent email spoofing. Mail transfer can happen over TCP port 25, 465, or 587. Unlike sending between servers, mail does not normally have to come from a certain server; instead, you authenticate using a password.