As diagnosed correctly by FeedBurner, the problem of Outlook 2007 not being counted in the RSS subscription statistics lays with Outlook. Michael Affronti, the Outlook Program Manager responsible for the RSS feature, just announced that Outlook 2007 doesn’t and won’t report a custom user-agent string. That means, any service, not just FeedBurner, trying to interpret the data will only see requests from IE7 that look like someone is browsing a feed. Without a custom user-agent string, there is simply no way to differentiate real IE7 feed browsing from an Outlook 2007 feed subscription.

This is extremely unfortunate and with Outlook 2007 probably going to be adopted widely, means that RSS feed subscription statistics will be significantly lower than their real number. How much lower? I can only guess.

FeedBurner reported that I had 148 subscribers and 2445 hits from IE7 on Monday. It seems to me most likely that the majority of those IE7 hits are generated by Outlook 2007. After all, most people would not browse to my RSS feed, but rather browse to my website. Let’s be conservative though and assume that only 80% of the hits (=1956) are generated by Outlook 2007. I don’t know how often Outlook pulls my feed, but once every hour sounds reasonable. If I am very conservative again, I assume that a copy of Outlook pulling my feed is running 24 hrs. In that case, the 1956 hits represent 81.5 subscribers. Looking at my weekend drop though, I would say it is reasonable to assume that most people read my blog at work or at least during office hours. So maybe they don’t have Outlook running 24 hrs, but just 16 hrs. That would represent 122.25 subscribers.

I get to pick then between my total subscription number being 229 or 270 instead of the reported 148.