Sending SMTP does not follow MX records correctly. If the lowest mx e.g. 10 is not available the proxy holds the email rather than sending it to the mx20 or 30 that were online.
Grey listing never seems to time out, docs say 5 minutes, waited 25 but logs still show waiting. The grey listing parameters should be configuarble - retention of 1 week is too short, 5 minutes may be needed in some situations. [:(]