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 heuristic 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 sends 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 re-attempt deliver in X number of minutes. (it depends upon their configuration).
If the sender mail server waits and re-delivers, 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's mail server doesn't reattempt the delivery (and the spammers usually do not re-attempt delivery) 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 (mail server sender) is not configured to re-attempt the delivery. If you see some valid email on your delaying log page, you can whitelist the email using the REPORT button.
