We're getting an error message when the forum server sends an e-mail. This happens when either (a) a new member registers and the server sends an e-mail confirmation, or more commonly (b) when you send a private message to someone and the server sends an e-mail to that person notifying them of the private message.
Registrations are being completed and the private messages are being sent, so you can ignore the error message. I'm trying to figure out what is causing it though. Those e-mails are not being sent, so what I'm thinking is that it's the server of the hosting service that is having a (temporary) problem. I don't like that e-mail server anyway because Yahoo thinks it is a spammer. Might try to change the configuration so the e-mails are sent from somewhere else.
Eric
Update, Nov. 14: This was a problem with some new filters the hosting service had installed on their server. The problem has been fixed so the notification e-mails are being sent again. They may still land in your spam filter though.
Eric