Greylisting

The mail server generating this error message is using a system called Greylisting to filter spam and viruses. It relies on Mail Servers behaving according to the standards and usually doesn’t require any action by the user. Your mails should only possibly be delayed.

If you see this error message while sending a mail, it probably means that you did something wrong: the configured mail server should trust your machine and thus not apply greylisting at all. Ask your system adminstrator if you are not sure about how to configure your mail client.

See this page for a description about how Greylisting works.