What is a 'Mail block - relay denied' error?

‘Relay’ refers to the transmission of an email from one server to another. Your email has bounced due to a temporary error, which could be on the sending or the receiving side. 
This type of bounce usually occurs:

• when the sender’s message is not authenticated, or 
• due to a misconfigured server on the recipient side.

To know more about Email Bounces and Error Codes, click here.