mail is not working

hello all
i'm using the command "mail" to send emails i'm using slackware 9.1
if u use this command and try to send an email, i get a returned email directly stating that spider.terranet doesn't exists
here is the returned mail:

----- Transcript of session follows -----
... while talking to atlantic.terra.net.lb.:
>>> MAIL From:<root@Spider.terranet> SIZE=434
<<< 553 Spider.terranet does not exist
501 5.6.0 Data format error


any ideas? thanks

Is spider.terranet defined in your /etc/hosts or do you have a local DNS server running? If neither of these are true, you'll need to put an entry for it in your /etc/hosts file so your MDA knows where to look. 8)