I think it's important to start by saying that using Lotus Notes only as an email client misses the bigger picture, as Lotus Notes was was so much much much more than that. IBM had thouthands of custom made apps running on Lotus Notes. I had built about 50 apps.
Also, you didn't mention when you used it. I worked with Lotus Notes from 1995 to 2010, and the experience really depended on the hardware available at the time. Once it was up and running, it performed just fine if you had enough memory. For example, I still have version 8.0.2 (from 2008) on my machine, and it uses roughly 13MB at idle and up to about 50MB depending on installed apps, which isn't much by modern standards. But back in 1995, when 8MB of RAM was common, if Notes took up 6MB, I can see how that would have been a real issue. In my case, my ThinkPad had 32MB, then 64MB, and even 192MB which is the moment I installed Windows 2000.
As for the UI, the email client was highly configurable. Honestly, I don't remember ever thinking that something like Netscape Mail or The Bat looked or any email client that I tried at the time looked better. I didn't use Outlook back then, so I can't compare directly, but if Outlook over the past decade is any indication, Lotus Notes was at least on par, if not better that it.
What's really surprising is that even today, Lotus Notes had features that many modern clients still lack. For example, you could select multiple emails and forward them all together in a single message. Simple, but incredibly useful, and not something you commonly see implemented even now.
For reference, here's what the email client looked like:
https://ds-infolib.hcltechsw.com/ldd/dominowiki.nsf/xpDocVie...
I still don't see how Outlook is better than that.