Facts about SMTP

1. 08

   Extended SMTP, known as ESMTP, introduced the EHLO command as a modern replacement for HELO, enabling servers to advertise supported extensions during connection handshakes.

   SMTPJun 8sharenetworkingemailprotocol
2. 07

   Bounce messages generated by SMTP failures are sent to the envelope sender address rather than the Reply-To header, a distinction critical for distinguishing delivery errors from user responses.

   SMTPMay 14shareemailtechnicalprotocol
3. 06

   During an SMTP transaction, the DATA command initiates a period where servers accept up to 1000 lines of message content before processing reaches completion.

   SMTPMay 14shareprotocoltechnicalmeasurement
4. 05

   The SMTP protocol requires a 25-character maximum for local usernames before the @ symbol, limiting email address complexity in early system designs.

   SMTPMay 14sharestandardsemailtechnical
5. 04

   SMTP uses the VRFY command to verify whether a recipient address exists on a mail server, though many administrators disable this feature to prevent spam harvesting attacks.

   SMTPMay 14sharesecurityprotocolemail
6. 03

   SMTP servers reject messages exceeding 998 characters per line, a limit defined in RFC 5321 to ensure compatibility across diverse email systems.

   SMTPMay 14sharetechnicalstandardsemail
7. 02

   Port 25 remains the standard transmission channel for SMTP, though modern servers increasingly use ports 587 and 465 to enhance security and reduce spam.

   SMTPMay 14sharetechnologynetworkingsecurity
8. 01

   In 1982, Jon Postel and Joyce K. Reynolds published RFC 821, establishing SMTP as the standard protocol for sending emails across the internet.

   SMTPMay 14shareprotocolhistorynetwork