--- a/mailnews/local/src/nsParseMailbox.cpp
+++ b/mailnews/local/src/nsParseMailbox.cpp
@@ -930,20 +930,21 @@ nsresult nsParseMailMessageState::StartN
   return ParseEnvelope (line, lineLength);
 }
 
 /* largely lifted from mimehtml.c, which does similar parsing, sigh...
 */
 nsresult nsParseMailMessageState::ParseHeaders ()
 {
   char *buf = m_headers.GetBuffer();
-  char *buf_end = buf + m_headers.GetBufferPos();
+  uint32_t buf_length = m_headers.GetBufferPos();
+  char *buf_end = buf + buf_length;
   while (buf < buf_end)
   {
-    char *colon = PL_strchr (buf, ':');
+    char *colon = PL_strnchr(buf, ':', buf_length);
     char *end;
     char *value = 0;
     struct message_header *header = 0;
     struct message_header receivedBy;
 
     if (! colon)
       break;