I'm working with a customer whose IT department states, inequivocably, that as part of our services, emails sent in behalf of this customer through our bulk server are never making it to their Exchange servers. I asked them which ISP and what filters. They mentioned their their ISP and that Postini is their filter.

But Postini is NOT receiving the email and throwing it into quarantine. Postini NEVER sees the mail.

Further, if the customer sends a dedicated campaign to their own 190 employees, the mail is received, 100% deliverability. Mix these same employees with 10K of their subscribers, and suddenly mail goes missing. Threshold? Where? Why wouldn't the customer's IT staff see mail server rejecting the mail and throwing it into quarantine? If a server rejects mail, why wouldn't that server keep that in the log? What happens when a filter like Postini sits outside of an ISP. Is that even plausable, or do filters always sit inside of an ISP service or firewall?

My question, finally (I'm out of breath)... what does a bulk mail flow/schematic delivery system look like. Bulk Mail -> MTA -> Filter -> ISP -> Mail server? How do the parts work? This is a pretty technical question on how bulk email is delivered, how and where filters work in the process, where problems occur... and who/how to solve them.

Any links that provide a good read on the topic is greatly appreciated.