The office transitioned from Postini to ProofPoint last month, and it sure feels like delivery times have slowed. We use Google Apps (Gmail) as our mail server. Here's what I've noticed thus far:
When I was conducting send/receive QA/QC after changing our MX records to the ProofPoint SMTP server, I noticed that while some emails where reasonably quick to appear in the recipient inbox (< 60 seconds), others were slow (> 5 minutes).
The slowness appears to be on the ProofPoint side as opposed to the Google Apps side. When a test email was slow to appear in the recipient's inbox, it was also slow to appear in the ProofPoint email log. Then, when it appeared in the inbox, it would appear in the log almost simultaneously.
I recognize that this evidence is far from definitive. A ProofPoint rep could reasonably argue that the slowness is attributable to the sending SMTP server.
I'd like to explore the issue further to determine if ProofPoint, is in fact, my speed bottleneck or if the issue is something "upstream" of ProofPoint. If ProofPoint is the issue, it would be nice to have some hard numbers to present to them.