The AllWorldIT email platform employs a form of fighting spam called greylisting. In name as well as working, greylisting is related to whitelisting and blacklisting. What happens is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later" message (This happens at the SMTP layer and is transparent to the end user). This, in the short run, means that all mail gets delayed atleast until the sender tries again, this is where spam loses out! Most spam is not sent out using RFC compliant MTAs (Mail Transfer Agent), the spamming software will not try again later.
If the MTA tries again after the greylisting period (4 minutes), the triplet (sender email address, senders server IP and recipient email address) is whitelisted for 7 days. If email from the same sender to the same recipient is received within 7 days it will not be delayed and will reset the whitelisting period for a further 7 days.
Various analysis tools are run to generate and use statistics from greylisting and may automatically blacklist some hosts exhibiting bad or abusive behavior.
Errors receiving mail?
If you are having trouble receiving mail and the error returned to the sender contains "Recipient address rejected: Policy Rejection: Greylisting - Please try later.", it means the senders mailserver must try again 4 minutes after its first attempt. Seeing as you're not receiving the email and the sender is getting an error it means that there is something wrong, the senders email server is not trying again. Please have the sender contact his ISP or email service provide to find out what the problem is.
The folowing RFC's describe in detail the retrying of temporarily rejected delivery attempts:
RFC 2821, page 42/3, section 4.2.1 describes error 45x as being temporary mail error.
RFC 2821, page 58, section 4.5.4.1 states the following "The sender MUST delay retrying a particular destination after one attempt has failed. In general, the retry interval SHOULD be at least 30 minutes;....".
References:
More information on Greylisting
SMTP Standard - RFC2821
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