I've been using Microsoft stuff (basically many components from the 365 suite, including outlook) for the last three months, including Outlook.
I must say, I am definitely impressed. I haven't rebooted my work laptop in nearly three months and it's been working tirelessly since then.
You might not like outlook, but it hasn't let me down.
I am impressed by what Microsoft is achieving.
If Microsoft gets pricing right and offers free tier and bot/api, slack is going to have a relly hard time...