If mass mailer is also failing then the mailer isn't configured properly (non-issue from CB Invites). Please ensure your Joomla configuration is absolutely correct. I suggest running tests with CB > User Management > Mass Mailer during configuration to get things working right. For example the below is SMTP configuration if using GMAIL.
Mailer: SMTP Server
SMTP Authentication: Yes
SMTP Security: SSL
SMTP Port: 465
SMTP Username: Your Google Username
SMTP Password: Your Google Password
SMTP Host: smtp.gmail.com