If someone tries to send you an email message and receives an error containing "451 Greylisted, please try again in 900 seconds", this usually means that they are sending from a non compliant mail server or that the settings on the sending server are not set to auto-retry failed messages as required by RFC specifications.
In some cases this may just be an informational warning from the senders server that the email message failed and that the sending server will retry sending the message. However, to be safe, you should request that the sender resend the message.
The reason this happens is that we have implemented Greylisting on our servers in order to combat spam. For information on Greylisting and how it works, please read this article: What is Greylisting
