The main thing this error message usually represents is a problem with the method of sending out e-mail messages that phpBB has been told to use. By default, phpBB will try to use a server feature called sendmail. When this error appears in all of the cases mentioned previously, it can mean that sendmail is not installed on the server, has been removed by a server "upgrade," is malfunctioning or the server is configured incorrectly for sendmail to be used. In any of these cases, there is not much you can do unless you are the server owner and/or administrator, in which case you likely know how to fix sendmail already. Assuming you are not the server owner/administrator, you can attempt to contact that person to find out if sendmail is available, configured and working correctly on the server. Some servers, most notably those running a version of the Windows operating system, may not have sendmail available. In these cases, you will need to switch your forum to an SMTP mailing service.
SMTP, for the many readers of this tutorial that will have never heard of it before, is basically a way of sending e-mail messages through a separate, remote server instead of the server hosting your forum. To switch the forum to SMTP, you can use some settings found near the bottom of the Admin Panel > General > Configuration page. You will have to fill in a server address, a username, a password and possibly other information. Your web host may provide this information if they make a SMTP mailing service available. If they have no such service, you will have to locate one on your own through Google
. Some internet service providers (ISPs) provide SMTP mailing services to their clients, so that may also be an option for you.