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.