Greylisting takes advantage of the fact that the send routine of spam mail will not further react in case of an error when spam is being delivered. A trusted mail server would try to deliver another time, not so most of the spammers! Caused by the huge number of email a spammer sends, correct error handling could overstress the existing infrastructure or would simply take too long. Another try to deliver will not take place, means, spam is blocked. Viruses with SMTP spreading routines behave the same way. They typically miss error handling too, so greylisting works as well to block that kind of malicious ware.

Based on its structure, the grey list only steps in when DEFENDO communicates directly with the sender of the unwanted email. So, as a basic requirement, DEFENDO has to receive incoming email via SMTP. In case of receiving email via POP3 from the ISP, greylisting will not work at all. Same applies to SMTP delivery via upstream relays, i.e. installed at the ISP's site.

As a matter of fact, greylisting acts at a very early juncture of the email transfer. Even before the transmission of the actual message, the connection will be shut down. That results in some good effects: The internet access will not be stressed and DEFENDO can relax since it does not have to scan for viruses and spam. If set up well, the efficiency of greylisting gets closed to perfection.

For the ones who want better performance, even the following easily structured feature will do the job. The greylisting system will only act, if the senders address is listed in DNS data bases of well known spammers and dynamic IP addresses. While regular email doesn't have to pass greylisting, unwanted email is subject to be blocked before the system. As a consequence, internet connection and DEFENDO will be released from traffic.

That new feature means significant advantages to customers who run domains accepting all recipient addresses (multi-drop or catch-all-domains) but also for internal mail servers without the capability to verify recipient addresses in advance (e.g. Exchange 2000 and older versions). In that environments greylisting will dramatically reduce the so called spam bounces or collateral spam. By collecting the sender addresses of outgoing mail, DEFENDO learns the addresses of active internal users. Inbound mail to these known internal users will be accepted immediately. All the remaining mail still is forced to pass the greylisting process. The number of undeliverable messages will decrease significantly and all systems involved will be released traffic wise.

Even for users who generally decide to go with greylisting, we have been able to speed up the communication process. DEFENDO's brand new greylisting feature takes care for email replies without delay. In order to realize that, the sender and recipient addresses of outbound emails will - in reversed order - be accepted inbound. All the other email still gets filtered by the greylist.