--- sylpheed/libsylph/quoted-printable.c.alt-qpfix	2005-09-13 12:18:44 +0400
+++ sylpheed/libsylph/quoted-printable.c	2006-05-21 22:01:00 +0400
@@ -142,6 +142,28 @@ gint qp_decode_q_encoding(guchar *out, c
 	return outp - out;
 }
 
+/**
+ * Check if the character can be represented directly in a Q-encoded word.
+ *
+ * @param	c	Tested character
+ *
+ * @returns	1 if the character can be represented directly, 0 if it must be
+ * 		hex-encoded.
+ *
+ * @warning	The argument is evaluated multiple times.
+ *
+ * @note	This macro uses the most restrictive set of allowed characters
+ * 		(described in RFC 2047, section 5, (3)), so the word encoded
+ * 		according to these rules should be usable in all contexts where
+ * 		an encoded word may appear.
+ */
+#define IS_QP_SAFE_CHAR(c)					\
+	( ((c) >= 'A' && (c) <= 'Z') ||				\
+	  ((c) >= 'a' && (c) <= 'z') ||				\
+	  ((c) >= '0' && (c) <= '9') ||				\
+	  ((c) == '!') || ((c) == '*') || ((c) == '+') ||	\
+	  ((c) == '-') || ((c) == '/') )
+
 gint qp_get_q_encoding_len(const guchar *str)
 {
 	const guchar *inp = str;
@@ -150,11 +172,10 @@ gint qp_get_q_encoding_len(const guchar 
 	while (*inp != '\0') {
 		if (*inp == 0x20)
 			len++;
-		else if (*inp == '=' || *inp == '?' || *inp == '_' ||
-			 *inp < 32 || *inp > 127 || g_ascii_isspace(*inp))
-			len += 3;
-		else
+		else if (IS_QP_SAFE_CHAR(*inp))
 			len++;
+		else
+			len += 3;
 
 		inp++;
 	}
@@ -170,13 +191,13 @@ void qp_q_encode(gchar *out, const gucha
 	while (*inp != '\0') {
 		if (*inp == 0x20)
 			*outp++ = '_';
-		else if (*inp == '=' || *inp == '?' || *inp == '_' ||
-			 *inp < 32 || *inp > 127 || g_ascii_isspace(*inp)) {
+		else if (IS_QP_SAFE_CHAR(*inp))
+			*outp++ = *inp;
+		else {
 			*outp++ = '=';
 			get_hex_str(outp, *inp);
 			outp += 2;
-		} else
-			*outp++ = *inp;
+		}
 
 		inp++;
 	}