If you are receiving this error after email to external destination email address this means, they have a spam blocking system which use Greylisting (or Delaying) method to stop spam.
This method is based in the idea that a spam server will not send you the same message more than once in a period of time.
Greylisting generates a temporary error on purpose and waits for the sending server to come back later and try to deliver the email again.
A normal server, waits a while, tries again and then succeeds in delivering the mail. A mass email engine (spammer) tries one time and then the email is lost forever.
The greylisting mechanism keeps a database of approved senders, each consisting of sender address, recipient address and the IP address of the sending email server. When a new combination is found for the first time, a message looking something like this is sent back to the sending email server:
451 4.7.1 Please try again later
If the sending email server tries again before the X minutes has passed, it will receive a temporary error again. After the X minutes has passed and the sending server tries again, the email will pass and the sender triplet will be registered as "whitelisted". That means that subsequent emails from the same sender to the same recipient will be delivered without delay.
