Hi, I am trying to automatically forward my mail, but it bounces back saying this:

The following message could not be forwarded to  simon@<mydomain>.com  for <myusername> @whiteboxrisk.com  

Technical details of the failure: 
This server employs greylisting as a means of reducing spam. Your message has been delayed and will be accepted later. 

How can I fix the problem?

Many thanks,

Simon