Having a big problem where Office 365 user emails sent to us are being delayed sometimes up to 6 or 7 hours. I think that the emails are being greylisted, but it’s hit or miss. I had the understanding that if an email is greylisted it will only be greylisted once if the sending server complies with the resend within the specified time period. This is happening with the same senders over and over.
Have you looked in the log files for that sender to see if they are in fact being greylisted?
You have complete control over how greylisting operates. I would check out what your settings are and make sure you are allowing enough time for the message to be resent. I am positive that it isn’t an issue with Office 365 as far as compatibility goes, but it is more likely with it receives a 321 “Try again later” error, it waits way too long to try again.