diff --git a/mozilla/mailnews/local/src/nsParseMailbox.cpp b/mozilla/mailnews/local/src/nsParseMailbox.cpp
index 860e49e491b..e4e92582021 100644
--- a/mozilla/mailnews/local/src/nsParseMailbox.cpp
+++ b/mozilla/mailnews/local/src/nsParseMailbox.cpp
@@ -1402,7 +1402,7 @@ int nsParseMailMessageState::FinalizeHeaders()
               }
             }
           }
-          substring = PL_strstr(content_type->value, "multipart/mixed");
+          substring = PL_strcasestr(content_type->value, "multipart/mixed");
           if (substring)
           {
             PRUint32 newFlags;