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Greylisting is an anti-spam method that some email servers use to automatically block email from your senders email address. It is designed to stop email sent by mass email engines that send out email spam communication. 

If an email is sent by mass email engines and is blocked the email engine will not re-try to send the email if there is an error - so the email does not get through to the recipient. 

Greylisting generates a temporary error - however if the email server sending the email re-tries to send the email again then it will normally be successfully delivered as the recipients email server.  

If an email is still not delivered as it has bounced back it would be worth asking your contact to add your senders email address to their whitelist so it is recognised as a safe sender.