University of California, Riverside

C&C Antispam


What is Graylisting?

Graylisting is a simple but effective spam reduction technique that attempts to differentiate spam from legitimate messages by temporarily rejecting messages from unrecognized senders. Mail from legitimate sources will be resent after a short delay, at which point it will be accepted and sent to the appropriate recipient. Spam, on the other hand, will typically not be resent, preventing it from entering UCR's e-mail system.

Graylisting is a tool designed to be used in concert with other methods of spam elimination. At UCR, other spam reduction techniques include spam-tagging and Realtime Blocking Lists.

Continue reading for a more detailed technical description of how Graylisting works.

Will graylisting cause me to not receive legitimate e-mail?

Although it is possible that a legitimate e-mail sender may have a mis-configured server that could cause e-mail to be temporarily blocked, Computing and Communications is taking steps to ensure that only unwanted e-mail messages are blocked by Graylisting. For example, C&C has added the e-mail domain names of all UC campuses to an automatic "allow list", which will prevent Graylisting of mail sent to UCR from any other UC campus.

Read more information on how Graylisting affects the delivery of legitimate e-mail.

Will graylisting slow down the delivery of e-mail?

The first time a message is received from a particular e-mail server, there is a short (typically 5-10 minutes) delay in delivering it to the intended recipient. Once the sending e-mail server has successfully resent a message, any mail sent from it will be passed through to the intended recipient without delay.

Interested in reading how Graylisting can affect mail delivery times?

Questions, Comments, Feedback

Graylisting is just one of several anti-spam initiatives that began to deploy during the Fall quarter 2005. C&C welcomes questions, comments, or feedback concerning Graylisting, the new Realtime Blocking List (RBL) service, the campus spam tagging system, or any other e-mail related service or support provided by C&C. Please e-mail your question, any comments, or feedback to Phyllis Franco (, Manager, Computer Support Group.

More Information 

General Campus Information

University of California, Riverside
900 University Ave.
Riverside, CA 92521
Tel: (951) 827-1012

Department Information

Computing & Communications
Computing & Communications Bldg.

Tel: (951) 827-4741
Fax: (951) 827-4541