Quoting of the MIME boundary when it contains special characters.

src/mimemail.erl

@@ -637,7 +637,31 @@ guess_best_encoding(Body) ->
 encode_parameters([[]]) ->
 	[];
 encode_parameters(Parameters) ->
-	[case binstr:strchr(Y, $\s) of 0 -> [X, "=", Y]; _ -> [X, "=\"", Y, "\""] end || {X, Y} <- Parameters].
+	lists:map(fun encode_parameter/1, Parameters).
+
+	encode_parameter({X, Y}) ->
+		case escape_tspecial(Y, false, <<>>) of 
+			{true, Special} -> [X, $=, $", Special, $"];
+			false -> [X, $=, Y]
+		end.
+
+    % See also: http://www.ietf.org/rfc/rfc2045.txt section 5.1
+	escape_tspecial(<<>>, false, _Acc) ->
+		false;
+	escape_tspecial(<<>>, IsSpecial, Acc) ->
+		{IsSpecial, Acc};
+	escape_tspecial(<<C, Rest/binary>>, _IsSpecial, Acc) when C =:= $" ->
+		escape_tspecial(Rest, true, <<Acc/binary, $\\, $">>);
+	escape_tspecial(<<C, Rest/binary>>, _IsSpecial, Acc) when C =:= $\\ ->
+		escape_tspecial(Rest, true, <<Acc/binary, $\\, $\\>>);
+	escape_tspecial(<<C, Rest/binary>>, _IsSpecial, Acc) 
+		when C =:= $(; C =:= $); C =:= $<; C =:= $>; C =:= $@;
+			 C =:= $,; C =:= $;; C =:= $:; 
+			 C =:= $/; C =:= $[; C =:= $]; C =:= $?; C =:= $=;
+			 C =:= $\s ->
+		escape_tspecial(Rest, true, <<Acc/binary, C>>);
+	escape_tspecial(<<C, Rest/binary>>, IsSpecial, Acc) ->
+		escape_tspecial(Rest, IsSpecial, <<Acc/binary, C>>).
 
 encode_headers(Headers) ->
 	encode_headers(Headers, []).
@@ -1380,6 +1404,27 @@ encode_quoted_printable_test_() ->
 		}
 	].
 
+encode_parameter_test_() ->
+	[
+		{"Token",
+			fun() ->
+				?assertEqual([[<<"a">>, $=, <<"abcdefghijklmnopqrstuvwxyz$%&*#!">>]],
+				    encode_parameters([{<<"a">>, <<"abcdefghijklmnopqrstuvwxyz$%&*#!">>}]))
+			end
+		},
+		{"TSpecial",
+			fun() ->
+				Special = " ()<>@,;:/[]?=",
+				[
+    				?assertEqual([[<<"a">>, $=, $", <<C>>, $"]], encode_parameters([{<<"a">>, <<C>>}]))
+					|| C <- Special
+				],
+				?assertEqual([[<<"a">>, $=, $", <<$\\,$">>, $"]], encode_parameters([{<<"a">>, <<$">>}])),
+				?assertEqual([[<<"a">>, $=, $", <<$\\,$\\>>, $"]], encode_parameters([{<<"a">>, <<$\\>>}]))
+			end
+		}
+	].
+
 rfc2047_decode_test_() ->
 	[
 		{"Simple tests",