[exim] Accepting mail from empty senders

I'm having a bit of trouble googling for this one because "<>" is not a
meaningful term to them.

Is it required that we handle emails with a sender of <> ? Are these
legitimate bounces, or only spam attempts?

Our mailserver is getting sent spam which has an empty sender (contains only
<>) , so I'm looking for an ACL check which will enable us to handle it
correctly.

But I don't want to do this if we SHOULD be accepting these emails.