The BNF from RFC 5322 defining parts of a valid internet message address

       addr-spec       =   local-part "@" domain

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

       dot-atom        =   [CFWS] dot-atom-text [CFWS]

       CFWS            =   (1*([FWS] comment) [FWS]) / FWS

       FWS             =   ([*WSP CRLF] 1*WSP) /  obs-FWS
                                              ; Folding white space

       WSP             =   SP / HTAB          ; white space

       obs-FWS         =   1*([CRLF] WSP)     ; As amended in erratum #1908

       ctext           =   %d33-39 /          ; Printable US-ASCII
                           %d42-91 /          ;  characters not including
                           %d93-126 /         ;  "(", ")", or "\"
                           obs-ctext

       obs-ctext       =   obs-NO-WS-CTL
       ccontent        =   ctext / quoted-pair / comment

       comment         =   "(" *([FWS] ccontent) [FWS] ")"

       dot-atom-text   =   1*atext *("." 1*atext)

       atext           =   ALPHA / DIGIT /    ; Printable US-ASCII
                           "!" / "#" /        ;  characters not including
                           "$" / "%" /        ;  specials.  Used for atoms.
                           "&" / "'" /
                           "*" / "+" /
                           "-" / "/" /
                           "=" / "?" /
                           "^" / "_" /
                           "`" / "{" /
                           "|" / "}" /
                           "~"

       specials        =   "(" / ")" /        ; Special characters that do
                           "<" / ">" /        ;  not appear in atext
                           "[" / "]" /
                           ":" / ";" /
                           "@" / "\" /
                           "," / "." /
                           DQUOTE

       quoted-string   =   [CFWS]
                           DQUOTE *([FWS] qcontent) [FWS] DQUOTE
                           [CFWS]

       qcontent        =   qtext / quoted-pair

       qtext           =   %d33 /             ; Printable US-ASCII
                           %d35-91 /          ;  characters not including
                           %d93-126 /         ;  "\" or the quote character
                           obs-qtext

       obs-qtext       =   obs-NO-WS-CTL

       obs-NO-WS-CTL   =   %d1-8 /            ; US-ASCII control
                           %d11 /             ;  characters that do not
                           %d12 /             ;  include the carriage
                           %d14-31 /          ;  return, line feed, and
                           %d127              ;  white space characters

       quoted-pair     =   ("\" (VCHAR / WSP)) / obs-qp

       VCHAR           =   %x21-7E            ; visible (printing) characters

       obs-qp          =   "\" (%d0 / obs-NO-WS-CTL / LF / CR)

       obs-local-part  =   word *("." word)

       word            =   atom / quoted-string

       atom            =   [CFWS] 1*atext [CFWS]

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

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

       dtext           =   %d33-90 /          ; Printable US-ASCII
                           %d94-126 /         ;  characters not including
                           obs-dtext          ;  "[", "]", or "\"

       obs-dtext       =   obs-NO-WS-CTL / quoted-pair

       obs-domain      =   atom *("." atom)

    NB For SMTP mail, the domain-literal is restricted by RFC5321 as follows:

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

       address-literal  = "[" ( IPv4-address-literal /
                        IPv6-address-literal /
                        General-address-literal ) "]"

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

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

       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.