WordPress MU

I keep having this error
"sendmail: Can't send mail: Mail must not have multiple From addresses Status: 302 X-Powered-By: PHP/4.4.4-8+etch2+spbutters1 X-totalblogs: 19 X-rootblog: http://k37.org/"
whenever somebody try to comment in my site. I talked to the hosting support and they told me that sendmail has been activated already and they told to use smtp authentication in wordpress..
I am a newbie, what is an smtp authentication ? where in the world I could edit that? and how..
thank you....

Google it.