How does work the Delaying filter (Greylisting) ?
Delaying is an additional method we provide you to block significant amounts of spam at the mailserver level, but without resorting to heavyweight statistical analysis or other heuristical approaches .This method is also called “Greylisting”.
Delaying works on the idea that a correctly configured SMTP server will always attempt re-delivery of an email message if it gets a soft failure.
How does it work exactly ? When someone send an email to our server (and you have the delaying filter enabled), it will return a 451 error (soft failure) which requests deliverly again later. If the sending mail server is correctly configured it will reattempt deliver in X number of minutes. (it depends upon his configuration) .
If the sender mail server waits and redelivers , the triplet (email address, domain,IP) gets whitelisted (delaying whitelist) and you’ll receive the email . When will you receive the email ? If the mail server is configured correctly you should receive the email after min minutes (default embargo time) and max hours (default wait time) . If the sender mailserver doesn’t reattempts the deliver (and the spammers usually do not reattempt the deliver) the email will be rejected after the wait time ( hours) and you’ll never receive the spam message.
May I lose some valid email ? Only if the sender (mailserver sender) is not configured to reattempt the deliver the email will be rejected. You can even whitelist the domains which you wish to override from Greylisting by whitelisting the sender domains/emails from your webmail and those senders would not be included in the greylisting.
Once the mail server retries the sender ip/domain is recorded in the whitelist of the server and any mails in next 30 days would not be delayed.
The feature works wonderfully and is used as a very effective anti spam technique. Remember you won’t lose any mails with this unless the sender server is not configured to retry, by default every server is designed and configued to retry the mail unless its a spammer’s server.