Normally, an authentication problem. But sometimes it's about the recipient's server blacklisting yours, or an invalid email address.
The specific error you receive should state the exact problem, but it may mean any of the following:
As in almost every 'fix', you'll want to ensure the email address is correct and properly spelled. Once confirming, ensure that your email user is properly authenticating (login and password). Then, make sure that your IP address is not blacklisted.