There are often messages that have no From: address in the SMTP-envelope. There is a Null Sender Disposition filter in the Spam Filter which can put all the null sender emails to user's quarantine manager. But all null-sender emails are not spam messages. There are few messages that you would like to get delivered.
The solutions is, you can set the Null Sender Disposition to Ignore and make custom filters that would process those null-senders and send only emails which are valid. For marking the messages valid, you can filter by the mail headers and put the trusted domains from where those emails are coming. To do this, open postini, go to your organization main page and select Spam filtering. Once there, set Null sender disposition to Ignore and hit save.
Now click on Content Manager and click on Add custom filter. Set the filter name and status to off. Enter Set Rules to Match: All rules. For first rule, select sender and select empty on other drop-down. For second rule, select header and then select matches a regex. Create a regex consisting of domains from which you are receiving null senders. For example: example.com|domain.com
Set Routing to deliver and bypass junk filters. Additionally you can quarantine it to some other mail address for monitoring.
In a similar way, create another filter which will blackhole all the emails when the sender is empty. Give the first filter(matches given regex for null senders) we made above greater priority than the second filter we made (blackhole null senders).
So, when a mail comes in, all the filters will applied according to the priority. The filter having a regex should be executed first than the filter we set the senders as empty.