Basic SMTP service has three different protocol-level variations:1. SMTP : Port 25 | Unencrypted, some servers support STARTTLS command, which allows the connection to be encrypted AFTER it has been established.2. SMTPS : Port 465 | Encrypted from the very first packet.3. Submission : Port 587 | virtually identical to SMTP, except that you must authenticate yourself (SMTP-AUTH) before submitting an email. ALSO Unencrypted and ALSO usually supports STARTTLS command to begin encrypting the connection after it has been established. It is handled by telling the sending application to use SMTP on an alternate port, 587.
Additional Info:----------------------- Port Authentication SSL EncryptedSMTP 25 x xSMTPS 465 v vSubmission 587 v x