You need to look at the SQL and SSRS error logs to see the actual error message. This is insufficient information for us to help you.
Just off the top of my head I'm going to guess your email server is rejecting the call because SSRS is sending the email using a From
address that isn't valid for your email server. Relaying is generally not turned on in mail servers for security reasons so the email address that the email is being sent from needs to be a valid email address on your mail server. But that is just guessing without knowing your configuration.