You came here probably after seeing a mail of yours being rejected.
The mail server generating this error message is using a system called Greylisting to filter spam and viruses send to our mail forwarding service. 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.
Greylisting operates by temporarily rejecting emails from unknown or suspicious sources, issuing a 4xx error code (e.g., "please call back later") as defined in the Simple Mail Transfer Protocol (SMTP). Compliant SMTP servers are programmed to retry message delivery after such temporary failures. Legitimate emails are typically delayed but eventually delivered upon subsequent attempts. This method effectively filters out spam, as many spam-sending servers do not attempt redelivery after an initial rejection.
In practice, when an email is received from an unfamiliar sender, the receiving server responds with a temporary rejection. A legitimate sending server will queue the email and attempt to resend it after a delay, adhering to SMTP standards. Upon the retry, the receiving server recognizes the sender's persistence and accepts the email. This process not only deters spam but also provides a window for real-time blackhole lists and similar mechanisms to update and flag potential spam sources before the email is finally accepted.