Is SPF records setup correctly? http://mxtoolbox.com/
IS the IP added to the SPF records?
Does the SPF contain hard fail which could prevent email delivery?
Has the server been changed recently e.g. moved to a different provider.
IS MX records setup correctly
Images in Emails.
If images are not displaying in Emails then it might means that “wrong SSL” is used or the receiver does not have CA of the sender.