gen_smtp_client: Add support for fallback to HELO.

src/gen_smtp_client.erl

@@ -394,11 +394,31 @@ do_AUTH_each(Socket, Username, Password, [_Type | Tail]) ->
 
 -spec try_EHLO(Socket :: socket:socket(), Options :: list()) -> {ok, list()}.
 try_EHLO(Socket, Options) ->
-	socket:send(Socket, ["EHLO ", proplists:get_value(hostname, Options, smtp_util:guess_FQDN()), "\r\n"]),
-	%% TODO handle fallback to HELO!
-	{ok, Reply} = read_possible_multiline_reply(Socket),
-	Extensions = parse_extensions(Reply),
-	{ok, Extensions}.
+	ok = socket:send(Socket, ["EHLO ", proplists:get_value(hostname, Options, smtp_util:guess_FQDN()), "\r\n"]),
+	case read_possible_multiline_reply(Socket) of
+		{ok, <<"500", _Rest/binary>>} ->
+			% Unrecognized command, fall back to HELO
+			try_HELO(Socket, Options);
+		{ok, <<"4", _Rest/binary>> = Msg} ->
+			quit(Socket),
+			throw({temporary_failure, Msg});
+		{ok, Reply} ->
+			{ok, parse_extensions(Reply)}
+	end.
+
+-spec try_HELO(Socket :: socket:socket(), Options :: list()) -> {ok, list()}.
+try_HELO(Socket, Options) ->
+	ok = socket:send(Socket, ["HELO ", proplists:get_value(hostname, Options, smtp_util:guess_FQDN()), "\r\n"]),
+	case read_possible_multiline_reply(Socket) of
+		{ok, <<"250", _Rest/binary>>} ->
+			{ok, []};
+		{ok, <<"4", _Rest/binary>> = Msg} ->
+			quit(Socket),
+			throw({temporary_failure, Msg});
+		{ok, Msg} ->
+			quit(Socket),
+			throw({permanent_failure, Msg})
+	end.
 
 % check if we should try to do TLS
 -spec try_STARTTLS(Socket :: socket:socket(), Options :: list(), Extensions :: list()) -> {socket:socket(), list()}.
@@ -686,6 +706,31 @@ session_start_test_() ->
 						end
 					}
 			end,
+			fun({ListenSock}) ->
+					{"retry with HELO when EHLO not accepted",
+						fun() ->
+								Options = [{relay, "localhost"}, {port, 9876}, {hostname, "testing"}],
+								{ok, _Pid} = send({"test@foo.com", ["foo@bar.com"], "hello world"}, Options),
+								{ok, X} = socket:accept(ListenSock, 1000),
+								socket:send(X, "220 \r\n"),
+								?assertMatch({ok, "EHLO testing\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "500 5.3.3 Unrecognized command\r\n"),
+								?assertMatch({ok, "HELO testing\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "250 Some banner\r\n"),
+								?assertMatch({ok, "MAIL FROM: <test@foo.com>\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "250 ok\r\n"),
+								?assertMatch({ok, "RCPT TO: <foo@bar.com>\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "250 ok\r\n"),
+								?assertMatch({ok, "DATA\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "354 ok\r\n"),
+								?assertMatch({ok, "hello world\r\n"}, socket:recv(X, 0, 1000)),
+								?assertMatch({ok, ".\r\n"}, socket:recv(X, 0, 1000)),
+								socket:send(X, "250 ok\r\n"),
+								?assertMatch({ok, "QUIT\r\n"}, socket:recv(X, 0, 1000)),
+								ok
+						end
+					}
+			end,
 			fun({ListenSock}) ->
 					{"a valid complete transaction without TLS advertised should succeed",
 						fun() ->