A recent design and spam test showed that Postini was the only filter that failed. The only details given were:
Postini - Filter set as default.
How can I find out what in this email is triggering postini's "default filter"?
Unfortunately only a few filters give us the specific reasons and Postini is not one of them. The best things to try at first are changing the from email, the subject and also try removing parts of the content to see if it's a particular paragraph that might be causing the filter to be tripped.
You might also want to setup the spf and domainkeys for the sending domain, if you have't already.
I have discussed with the client a couple times adding the spf records- and it is something they are unwilling to do.
We have discussed setting up a separate domain for their email marketing that my company controls, would that be a good work-around?