Maybe different now—back then, nearly everyone only had a single Exchange account. We looked at supporting both WebDAV and EWS. The result would have introduced by confusing UX but also reduce reliability of both solutions (adds a lot of testing complexity). Instead, we did a release of Entourage with WebDAV (a horrible protocol that Exchange barely ever supported) and a free update with just EWS support (also rough initially, but much better designed and fuller-featured).

When we got to building Outlook, we decided to look forward—Cocoa and EWS only, building a strong base so that future releases could be far more capable. When I started on Entourage, the team was executing on a strategy to shove Exchange capability into a consumer-oriented app. It was shaky from the start, there were so many problems and customer complaints. I gradually learned as a PM that often a more impactful but riskier product strategy is to go big—instead of fixing 50 small issues a week at a time, fix 1 big issue that takes a year but renders the 50 irrelevant.