from: <> ???

[mailing]

Hello, I've been using qmail for a few months now (with multiple domains) we have an exchange server (I didn't choose it, i promis) which routes all outgoing mail via qmail.      In the log I've seen a few messages refused from the destination host(on the internet) because the from line is invalid. in fact the from line contains FROM: <>   . Does anyone know what this, what it means and why..?       Thanks in advance       Regards      Ken