From: DeskNow Knowledge Base
http://www.desknow.com/kb/

450 Recipient address rejected: Try again later
http://www.desknow.com/kb/idx/11/192/article/

This is a common response message coming from mail servers that implement an antispam measure called greylisting.

The server rejects a connection the first time. A spammer will not bother retrying later, but honest servers will. The second connection will be accepted. For more details, see http://en.wikipedia.org/wiki/Greylisting


As stated in the message, DeskNow will retry later, and the email delivery will typically succeed (if it does not, DeskNow will try again a few times, and if even the last attempt fails, DeskNow will send a second bounce message stating that the error is permanent).

If you do not want to receive these warnings, you can disable them by unchecking the option Administration/Mail/SMTP Outgoing/Delivery attempts/Send delivery error notification for first attempt.