This section covers both RFC-821 and RFC-822.
The SMTP specification in RFC-821 is clear and contains numerous examples, so implementors should not find it difficult to understand. This section simply updates or annotates portions of RFC-821 to conform with current usage.
RFC-822 is a long and dense document, defining a rich syntax. Unfortunately, incomplete or defective implementations of RFC-822 are common. In fact, nearly all of the many formats of RFC-822 are actually used, so an implementation generally needs to recognize and correctly interpret all of the RFC-822 syntax.