Document Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in RFC 2119. As each

of these terms was intentionally and carefully chosen to improve the

interoperability of email, each use of these terms is to be treated

as a conformance requirement.

Because this document has a long history and to avoid the risk of

various errors and of confusing readers and documents that point to

this one, most examples and the domain names they contain are

preserved from RFC 2821. Readers are cautioned that these are

illustrative examples that should not actually be used in either code

or configuration files.

The SMTP Model