+This SquirrelMail version added http headers that prevent caching of pages by
+proxies. Headers are added in SquirrelMail displayHtmlHeader() function. Changes
+require that html output is not started before displayHtmlHeader() is called. If
+some code starts output, PHP errors will be displayed. If plugins display
+notices in options_save hook and don't stop script execution on error, page
+display will be broken.
+
+SquirrelMail 1.5.1 implemented code that unregisters globals in PHP