The following summarizes the differences between this standard and the one specified in Arpanet Request for Comments #733, "Standard for the Format of ARPA Network Text Messages". The differences are listed in the order of their occurrence in the current specification.
These now must be a sequence of printable characters. They may not contain any LWSP-chars.
The characters period ("."), left-square bracket ("["), and right-square bracket ("]") have been added. For presentation purposes, and when passing a specification to a system that does not conform to this standard, periods are to be contiguous with their surrounding lexical tokens. No linear-white-space is permitted between them. The presence of one LWSP-char between other tokens is still directed.
Atoms may not contain SPACE.
ctext and qtext have had backslash ("\") added to the list of prohibited characters.
The lexical tokens <domain-literal> and <dtext> have been added.
The "Return-path:" and "Received:" fields have been specified.
The "From" field must contain machine-usable addresses (addr-spec). Multiple addresses may be specified, but named-lists (groups) may not.
The meta-construct of prefacing field names with the string "Resent-" has been added, to indicate that a message has been forwarded by an intermediate recipient.
A message must contain at least one destination address field. "To" and "CC" are required to contain at least one address.
The field-body is no longer a comma-separated list, although a sequence is still permitted.
The field-body is no longer a comma-separated list, although a sequence is still permitted.
A field has been specified that permits senders to indicate that the body of a message has been encrypted.
Extension fields are prohibited from beginning with the characters "X-".
Fewer optional forms are permitted and the list of three-letter time zones has been shortened.
The use of quoted-string, and the ":"-atom-":" construct, have been removed. An address now is either a single mailbox reference or is a named list of addresses. The latter indicates a group distribution.
Group lists are now required to to have a name. Group lists may not be nested.
A mailbox specification may indicate a person's name, as before. Such a named list no longer may specify multiple mailboxes and may not be nested.
Addresses now are taken to be absolute, global specifications, independent of transmission paths. The <route> construct has been provided, to permit explicit specification of transmission path. RFC #733's use of multiple at-signs ("@") was intended as a general syntax for indicating routing and/or hierarchical addressing. The current standard separates these specifications and only one at-sign is permitted.
The string " at " no longer is used as an address delimiter. Only at-sign ("@") serves the function.
Hierarchical, logical name-domains have been added.
The local-part "Postmaster" has been reserved, so that users can be guaranteed at least one valid address at a site.