A new anti-spam measure called MailHurdle is currently being tested on the VTMail system. MailHurdle is based on a commonly-used technology called greylisting. For more information on greylisting, see Wikipedia's Greylisting article.
While the tests are running, email arriving from previously unseen sources will be delayed by a temporary delivery error. For example, if the user receives a 451 error code, that user need not worry about their message going through, as the system will automatically resend the message after a given period of time. Ninety-nine percent of spam-generating software will view any delivery error as a fatal problem and will not attempt to resend the message. Legitimate, standards-compliant mail systems will properly recognize this temporary error and resend the message, usually within 30 minutes. Upon this second attempt at sending, the message is immediately allowed through the system to reach its recipient.
After the email arrives successfully, subsequent messages from that particular sender, over the same sending system, and to the same recipient will arrive without delay for a month following the last successful message transmission; each time a message is sent, this month-long time period automatically restarts.
Some legitimate mail systems are not standards-compliant; MailHurdle maintains a master list of these that is updated every night. Any system on the list will not be delayed, even if it is sending spam.
Note: MailHurdle and spam ranking are two separate processes; MailHurdle does not affect our spam ranking.
For more information on spam prevention, see Unwanted Email at Virginia Tech. If you fail to receive legitimate mail that you know has been sent, please contact 4Help using the Help Request Form or at (540) 231-HELP (4357).