I administered messaging and groupware systems for over 15 years. It was a large chunk of my career. I worked with both Exchange Server/Outlook and Lotus Domino/Notes, and generally preferred Lotus Domino - but both had their issues.
This article doesn't feel like it was written by someone who was using these platforms, nor like anyone was there at the time was consulted.
For example, focusing on F5 is odd. F5 is not the Office refresh key. F9 is. Excel uses it to recalculate. Word uses it to refresh fields. Outlook should have used it to fetch/refresh mail, but for some reason they went with the Windows/IE platform teams' choice of F5. Microsoft did actually switch back to using F9 in Outlook, and there was a significant outcry from their users. None of this is mentioned.
Similarly there's a screenshot of a messagebox stating that there are too many windows. No context given, but I'd bet it's from Loutus Notes 4.6 or earlier, which used MDI. MDI is limited to 9 windows. I hit that limit in a lot of applications back in the late WIndows 3.1/early Windows 95 days, at the peak of MDI. Notes Release 5 moved away from MDI, and gave the user a tabbed interface. The Exchange Mail and Outlook clients never used MDI, opening a new window for each mail - which may or may not be a better solution, given that Windows didn't have task stacking on the taskbar at the time. Nine tabs in Notes was just one taskbar entry, nine windows with Outlook was heading towards a serious usability problem...
I'd like to note that the Notes client didn't benefit from the network effects that Outlook did. At the time many of the complaints in the article were being made software packaging and deployment was in its infancy. Many environments made Notes and Outlook/Office part of their base build making updates a difficult thing to accomplish. Not to mention that the installers were quite large, taxing many networks of the time. But Outlook had the benefit of being part of Office - sometimes people wanted the latest features in Word or Excel, so the effort was made to update the installation. Notes had no such advantage. A similar network effect arrives when it comes to licensing, given that larger organisations will have volume licensing deals.
By contrast on the server side Exchange Server was a complete PITA to upgrade. You don't merely install new software, you build an entire new infrastructure and migrate mailboxes over to it. Very expensive, especially in the days of physical hardware. Whereas Lotus Notes Server/Domino was very easy to upgrade or extend.
As a result many Notes installations were up to date on their backend but not the frontend, whereas many Outlook installations were the other way around.
Anyway, I've gone on for too long - much like all the products we're discussing here. ;-)