Something changed in the past few weeks for me where email from gmail servers is getting more and more delayed (days at a time) and now some mails are never getting delivered (timed out on sending side I assume). What techniques are you using the accept easier or sooner mails from servers like mail-wm0-f66.google.com, mail-io0-f196.google.com, and on and on? (Normal greylisting simply won't work except by luck.)
Message
gmail delays
2016-04-12 by Jeremy C. Reed