[Exim] frozen without address

Hi,
I'm using exim 3.35 on my debian system, with amavis and a virus scan
engine. It happens that I find many messages in the queue like this:

53d  2.2K 18ybO6-0004Nw-00 <> *** frozen ***
          email@???

what does the absence of the sender address means ? Is there a way to avoid
this ?

--

Bye Enrico - Le finestre sono spiragli di finta liberta'. Scegli Linux.

You look like a million dollars. All green and wrinkled.