It can as well be Google, just the paid Apps version. Zero time to get used to a different UI. I suspect there must be a solution to easily migrate all your tags and filtering rules. (Tags are the killer feature to me. Outlook sort of has them but they are less flexible.)

does the paid apps version have better uptime? Is it not affected by the current issues?

My company has paid apps, and we have been facing issues same as everyone else.

I switched to a custom domain only when gmail torpedoed one of my secondary gmail accounts.