In an effort that started in 1990, approximately a decade after RFC 821 was completed, the PROTOCOL was modified with a "service extensions" model that permits the CLIENT and server to AGREE to utilize shared functionality beyond the original SMTP requirements.

The SMTP extension mechanism defines a means whereby an extended SMTP client and server may recognize each other, and the server can inform the client as to the service extensions that it supports.

The extension framework CONSISTS of:

• The SMTP command EHLO, superseding the earlier HELO,
• a registry of SMTP service extensions,
• additional parameters to the SMTP MAIL and RCPT commands, and
• OPTIONAL replacements for commands defined in this protocol, such as for DATA in non-ASCII transmissions.

In an effort that started in 1990, approximately a decade after RFC 821 was completed, the protocol was modified with a "service extensions" model that permits the client and server to agree to utilize shared functionality beyond the original SMTP requirements.

The SMTP extension mechanism defines a means whereby an extended SMTP client and server may recognize each other, and the server can inform the client as to the service extensions that it supports.

The extension framework consists of: