Debug_level

Can someone please tell me the correct way to use the debug_level option. I have tried setting it to $rcmail_config['debug_level'] = 1; and it does not put anything in the logs so I tried $rcmail_config['debug_level'] = 8; and also 4 and it only says "Failed to send message" trying to send a message via the web interface. This was working and then all of sudden stopped. It is possible that my ISP changed something, but the debug_level should show me what is going right? If not, can someone please tell me the troubleshooting steps?

thanks, Newbie to forum