I don't know why people like gmail so much but I found gmail's design, or Google's Material language in general, really bloated. Outlook.com has a flat and simple layout. Of course it's inferior to gmail in search and I don't understand why they makes me type out the "@outlook.com" part at login.
Another reason I use outlook is that I already gave everything else to Google so just diversify a bit.
Another reason I use outlook is that I already gave everything else to Google so just diversify a bit.