Postifx and virtual users (user unknown)

[redhatdude at bellsouth.net]

Why am I getting this message when someone sends me an email?
fatal: no SASL authentication mechanisms
I'm not trying to relay through postfix for which I should need  
authentication. I'm just sending mail to recipient. It should not  
check for authentication.

What does this message mean?
warning: /usr/libexec/postfix/smtpd: bad command startup -- throttling

> The cyrus-sasl-md5 rpm is installed? cyrus-sasl-plain is needed for
> supporting PLAIN and LOGIN.

I tried yum install cyrus-sasl-md5 and it says that there's nothing  
to do. Doesn't it get installed with cyrus-imapd?

Thanks
EJ