Sending mail from PHP - is the error at my end, or the Mail Server's?
If you are using the codeigniter mail object, try:$this->email->print_debugger()
Brute force method, go up one level from application and put in log_message/trace,etc, to debug the actual codeigniter code to see what is going on.
I had the same problem. I have set everything fine and got the same message.
The problem was in the fact that I have set to address with $this->email->to before I have applied smtp settings with $this->email->initialize($config). After I moved this line before all other settings, everything worked as it is supposed.