The error message you receive after the unsuccessful mail will usually tell us a lot about what could be happening. In most cases this is either becuase the email address in invalid, is full or the receiving party has blocked a patricular domain or IP address via their Spam Authority.
Check your email or junk mail folder for the bounce back message. Many of these use standard error codes to easily convey the possible cause of the error. I've attached a link to some very common codes.