The BNF from RFC 5321 defining parts of a valid SMTP address

       Mailbox        = Local-part "@" ( Domain / address-literal )

       Local-part     = Dot-string / Quoted-string
                      ; MAY be case-sensitive

       Dot-string     = Atom *("."  Atom)

       Atom           = 1*atext

       Quoted-string  = DQUOTE *QcontentSMTP DQUOTE

       QcontentSMTP   = qtextSMTP / quoted-pairSMTP

       quoted-pairSMTP  = %d92 %d32-126
                        ; i.e., backslash followed by any ASCII
                        ; graphic (including itself) or SPace

       qtextSMTP      = %d32-33 / %d35-91 / %d93-126
                      ; i.e., within a quoted string, any
                      ; ASCII graphic or space is permitted
                      ; without blackslash-quoting except
                      ; double-quote and the backslash itself.

       Domain         = sub-domain *("." sub-domain)

       sub-domain     = Let-dig [Ldh-str]

       Let-dig        = ALPHA / DIGIT

       Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig

       address-literal  = "[" ( IPv4-address-literal /
                        IPv6-address-literal /
                        General-address-literal ) "]"
                        ; See Section 4.1.3

       IPv4-address-literal  = Snum 3("."  Snum)

       IPv6-address-literal  = "IPv6:" IPv6-addr

       General-address-literal  = Standardized-tag ":" 1*dcontent

       Standardized-tag  = Ldh-str
                         ; Standardized-tag MUST be specified in a
                         ; Standards-Track RFC and registered with IANA

       dcontent       = %d33-90 / ; Printable US-ASCII
                      %d94-126 ; excl. "[", "\", "]"

       Snum           = 1*3DIGIT
                      ; representing a decimal integer
                      ; value in the range 0 through 255

       IPv6-addr      = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp

       IPv6-hex       = 1*4HEXDIG

       IPv6-full      = IPv6-hex 7(":" IPv6-hex)

       IPv6-comp      = [IPv6-hex *5(":" IPv6-hex)] "::"
                      [IPv6-hex *5(":" IPv6-hex)]
                      ; The "::" represents at least 2 16-bit groups of
                      ; zeros.  No more than 6 groups in addition to the
                      ; "::" may be present.

       IPv6v4-full    = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal

       IPv6v4-comp    = [IPv6-hex *3(":" IPv6-hex)] "::"
                      [IPv6-hex *3(":" IPv6-hex) ":"]
                      IPv4-address-literal
                      ; The "::" represents at least 2 16-bit groups of
                      ; zeros.  No more than 4 groups in addition to the
                      ; "::" and IPv4-address-literal may be present.