Greylisting (or graylisting) is a method of spam protection. A mail server using greylisting temporarily rejects any email from a sender it does not recognize. If the originating server is areal email server it will rety to send the email after a short delay. The receiving server will accept the email on the next attempt. Spammers often use poorly scripts that do not retry. In this way most spam from bot-nets is avoided. The only disadvantage of greylisting is that there is ashort (configurable) delay for incoming email from new senders. This can be as short as 10 minutes.
To add greylisting features to Exchange server you can use a tool such as Hexamail Guard
This allows greylisting options to be easily added to an existing Microsoft Exchange or Small Business Server, with lots of flexible options controlling and managing the greylist and clear and concise logging.
Hexamail is the only product that allows the greylist to be optionally used depending on the country of origin and time of day of the email. For example you may want to only greylist email during the night, so daytime email are not delayed in any way, and over night all email is greylisted when a short delay is of no consequence.
Similarly you can greylist only email from other countries. If most of your customers are in your timezone then the delay to email from other countries will not be noticeable:
Once greylisted a sender’s email will travel unimpeded through to the mailserver.