rfc2822 April 2001

3.4.1. Addr-spec specification

   An addr-spec is a specific Internet identifier that contains a
   locally interpreted string followed by the at-sign character ("@",
   ASCII value 64) followed by an Internet domain.  The locally
   interpreted string is either a quoted-string or a dot-atom.  If the
   string can be represented as a dot-atom (that is, it contains no
   characters other than atext characters or "." surrounded by atext
   characters), then the dot-atom form SHOULD be used and the
   quoted-string form SHOULD NOT be used. Comments and folding white
   space SHOULD NOT be used around the "@" in the addr-spec.

addr-spec       =       local-part "@" domain

local-part      =       dot-atom / quoted-string / obs-local-part

domain          =       dot-atom / domain-literal / obs-domain

domain-literal  =       [CFWS] "[" *([FWS] dcontent) [FWS] "]" [CFWS]

dcontent        =       dtext / quoted-pair

dtext           =       NO-WS-CTL /     ; Non white space controls

              %d33-90 /       ; The rest of the US-ASCII
              %d94-126        ;  characters not including "[",
                              ;  "]", or "\"

   The domain portion identifies the point to which the mail is
   delivered. In the dot-atom form, this is interpreted as an Internet
   domain name (either a host name or a mail exchanger name) as
   described in [STD3, STD13, STD14].  In the domain-literal form, the
   domain is interpreted as the literal Internet address of the
   particular host.  In both cases, how addressing is used and how
   messages are transported to a particular host is covered in the mail
   transport document [RFC2821].  These mechanisms are outside of the
   scope of this document.

   The local-part portion is a domain dependent string.  In addresses,
   it is simply interpreted on the particular host as a name of a
   particular mailbox.