SmarterMail 4.x Help
This documentation is for a product that is no longer supported by SmarterTools.


What is Greylisting and how does it work?

Greylisting is a new tool in the fight against spam. What it does is temporarily block incoming mail from a sender. It returns the mail to the sender's mail server with a message saying try again later. The sending server must then retry sending the mail after the Block Period but before the Pass Period(see below for definitions of these values).

Greylisting is effective because spammers will not usually bother to attempt a second delivery, but legitimate e-mail servers will.

Why use Greylisting?

Greylisting is a very effective method of spam blocking that comes at a minimal price in terms of performance. Most of the actual processing that needs to be done for Greylisting takes place on the sender's server. It has been shown to block upwards of 95% of incoming spam simply because so many spammers don't use a standard mail server which will do automatic retries.

How do I set up Greylisting?

Note: You must be a system administrator to change Greylisting settings.

In order to set up Greylisting go to the the Greylisting page in the Security menu.

  • Block Email with Greylisting - If this is enabled it will allow Greylisting to happen
  • Allow Users to Override Greylisting - If checked, this will allow users to selectively turn off Greylisting (useful if you have an account that receives time sensitive mail)
  • Block Period - The period of time(in minutes) that mail will not be accepted (default 15)
  • Pass Period - The period of time(in minutes) in which the sender's mail server has to retry sending the message (default 360)
  • Record Expiration - The period of time(in days) that the sender will remain immune from greylisting once it has passed (default 36)
  • Apply Greylisting to SmartHosts - If this feature is enabled it will determine whether or not SmartHosts are governed by Greylisting.

Disadvantages of Greylisting

The biggest disadvantage of greylisting is the delay of legitimate e-mail from servers not yet verified. This is especially apparent when a server attempts verify a new user's identity by sending them a confirmation e-mail.

Some e-mail servers will not attempt to re-deliver e-mail or the re-delivery window is too short. Whitelisting can help resolve this.