I had a problem with Greylisting and GWIA. Some e-mails were failing to get through, with this error message:
16:32:52 3C4 DMN: MSG 728 Send Failure: 451 4.7.1 message delayed, see http://www.greylisting.org ( http://www.greylisting.org/ ) ; Please try again later
The solutions was to adjust the GWIA SMTP/MIME settings for the “Intervals to retry a deferred message”. The default GWIA settings were:
I changed them to:
That fixed the problem.