You came here probably after seeing a mail of yours being rejected with a message like this:
Greylisted for 300 seconds (see https://rightvision.de/tools/postgrey)
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.
Make sure that you did turn on encryption and authentication for the SMTP server in your mail client. Ask your system administrator if you are not sure about how to configure it.
See this page for a description about how Greylisting works.