Mails from=<>

Hi,

Running Postfix 2.2 on RH Enterprise 4

I'm some kind of vacation-stand-in and not jet very familar with this
whole (productive) system.

There is a bunch of messages in the log like this:
postfix/nqmgr[16690]: 85D385FC51: from=<>, size=5367, nrcpt=1 (queue active)

Those mails don't have smtpd-entrys so I can't even say where they come
from and don't know how to with-read them.
Just to know if this machine is hijacked or something...

Maybe somone can help a bit understanding this.
I'm a bit confused...

Greetings
Nils