Outbound SMTP failure

When sending an SMTP mail via blat or telnetting into port 25 on the exhcnage server, mail does not reach external email addresses if you use a from address that isnt specified on that server, it reaches internal mail addresses though. If you use a from address that is valid on that server it works. Basically I thought that the from address had no bearing on the delivery of the email. Can someone explain or point me to a decent explanation of authenitcation when using exhcange SMTP mail?

Thanks

Steve