When sending emails in NetCommunity, we use specific techniques to limit spam. This solutions explain the use of a gray list and how this is incorporated within NetCommunity.
In order to minimize the amount of spam received by those who have email forwarding addresses, we use a number of anti-spam technologies including Graylisting. Essentially Graylisting prevents new senders from sending email to a recipient at the first attempt.
The first time an email from a new sender is submitted, the Graylisting service responds with a temporary error (451 4.7.1 Graylisted, Come Back Later) which causes most email systems to retry delivery after some delay. The next time the sending server attempts delivery the Graylisting service will authorize the sender and our server will accept the message (as long as the attempt occurs between 5 minutes and 6 hours later). The Graylisting authorization is valid for 15 days before the sender has to be re-authorized.
Graylisting is a very common practice for preventing spam and works quite well spam because many spammers will only attempt delivery 1 time