As part of our efforts to reduce SPAM use use a technique called Greylisting.

Greylisting is one of the most successful spam filtering techniques available. Greylisting can block as much as 70% to 80% of all SPAM that is sent to your email accounts.

Greylisting relies on a SPAMMERS reluctance to configure their mail servers according to internet standards by queuing and retrying message delivery.

How does Greylisting work?

With Greylisting, a record of three pieces of information when an email is received:

  1. The IP address of the machine sending the e-mail.
  2. The e-mail address of the person sending the e-mail.
  3. The e-mail address to which the e-mail is being delivered.

When a new domain or IP address tries to send email, we reply to the sending server and say “Greylisting in effect. Please try again to send this e-mail in 5 minutes.” The error message (called a “400-level error”) is specifically “temporary” and properly configured mail servers will queue the message and retry after some period of time.

After an administratively configurable period (default is 5 minutes), IF and WHEN the delivery is attempted again by the sending email server, the email server will attempt to match the information that was collected previously and if it matches the email is delivered without delay. From that point on, anytime a message with the matching information is received it is delivered immediately.

Why does greylisting work?
According to the internet specification, when a mail server receives a “400-level” error, it must queue the email message and try later to deliver it. For legitimate email, this process is standard and mandatory. Properly configured mail servers will redeliver their messages appropriately and Greylisting should not represent a delivery challenge to them. Because SPAMMERS send a huge number of emails per-day, they have no idea if they are valid or not, they generate a large number of bounced (non-delivered) messages. Acknowledging server responses for these messages, storing the messages on a server for some period of time, and redelivering them again represents, for a SPAMMER, a resource-intensive process. As a result, they intentionally misconfigure their mail servers not to retry delivery. Since Greylisting requires that an incoming email message originate from a properly configured mail server, most SPAM is filtered by Greylisting today.

Will my email be delayed?
Email affected by Greylisting will be delayed a minimum of 5 minutes. The delay interval must be long enough in order to prevent immediate redelivery by already connected SPAM SMTP servers.

Internet specifications suggest that messages temporarily refused be redelivered within 4 hours, and most servers are configured to retry in far less time – often on the order of 5-15 minutes. The specific delay will depend on the configuration of the sender’s email servers. Some email servers will wait anywhere from 15 minutes to 4 hours to retry delivery. This is a misconfiguration on the sending server side. Our servers will accept the retry after just 5 minutes.

Does Greylisting block some email?
As with any protection technology, nothing is 100%. There are cases where a poorly configured email server on the senders side can cause email not to be received. Lots of these issues came up when Greylisting was new. However now that’s it’s been in the field for some time, this is far less likely to happen. we maintain a list of known domains and email servers which have trouble with Greylisting and exclude them automatically.

What do I do if Greylisting is causing a problem for a specific sender?
You can submit a Support Ticket with their email address and we will exempt them from the Greylist system so their email will be accepted right away.