Greylisting is one of the methods used at CS to block spam. It is automatically enabled for everyone with a CS email address.
Greylisting works by delaying delivery of email that comes to us from unknown or untrusted sources. We maintain a list of common known and trusted sources. Additional non-spamming sources not in our known/trusted source list will prove they can be trusted by continuing to attempt delivery until the CS server accepts the email. Spammers almost never retry delivery, so most spam is prevented from getting to the CS email account.
Because only unknown and untrusted sources are delayed, most people with CS email accounts will not see any effect on their mail (other than a reduced amount of spam). In some rare cases, a legitimate message may be delayed. If you experience problems, please email the CS IT Support staff at email@example.com for assistance.
If a message has been delayed by greylisting, it will contain an X-Greylist header which will give some additional information about the delay.