Re: The Terrible...
Why, exactly is Outlook horrible?
What are some things that Outlook still gets wrong?
Well, let's see. It's astoundingly slow at searching "folders" - far slower than grepping through a bunch of mbox-format files would be (and mbox is a lousy idea created to fix the problem of poor allocation policies in old BSD filesystems). There's still no regular-expression search, or even decent Boolean queries. Searching a tree of folders requires toggling multiple UI controls, because that's never a thing that a user will want to do. Though it hardly matters, because nine times out of ten Outlook shows hits from outside the specified search scope anyway.
It still - still - renders some non-text media by default in the preview and reading views. That's been a whopping security hole since the late '90s. (Just respecting the sender's choice of fonts is suspect, given the number of font-rendering bugs in Windows - not all of them Microsoft's fault.)
The "attachment safety" mechanism is idiotic. It's trivially defeated by an attacker, but users who know what they're doing can't override it if they're in a domain and the admin has gotten clever. So people waste time working around it.
No PGP/GPG support. Yes, S/MIME does the same things; but it's much, much less widely used. (And the PGP/GPG PKI, while an unfriendly mess, is not as eye-wateringly stupid as the X.509 PKI used by S/MIME.)
Splitting large message stores apart, for example to improve backup time, is a pain in the ass.
Too many aspects of Outlook are a black box, and too many others are only apparent to the cognoscenti. Yes, you can delete an unsent message receipt using MFCMapi and suitable magical incantations - how many people know how to do that? Why not just put the outbound receipts in the outbox like everything else? Because Outlook was designed by people who Know Better Than You, so fuck you, user.
That's just off the top of my head.
I've used many, many MUAs, on PCs and workstations and minis and mainframes. Most of them have been pretty much crap. The real distinguishing characteristic: those that weren't impenetrable black boxes were consistently less of a pain in the ass. Outlook sucks because the developers have decided they know what users need to do, and that's what they're going to support. If other stuff doesn't work well, too bad.