you came here probably after seeing a mail of yours being rejected with a message like this:
Greylisted for 300 seconds (see http://www.kluenter.de/greylisted)
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 your provider did something wrong. If you need any help, send a Mail to firstname.lastname@example.org.