Zoom has huge distribution through their existing install base.
But so did Amazon when they launched their business suite, Workmail [0].
Email is one of the things you realllly don't want to move once you choose a solution. I can't imagine established organizations voluntarily migrating away from Gsuite or O365 to Zoommail (or Workmail)
And for new businesses owners, signing up for a business email generally comes before signing up for a video call service. Microsoft and Google both know this, which is why they both resell their suite through GoDaddy, NameCheap, and hundreds of other domain registrars and hosting companies.
Any speculation on how Zoom can succeed where Amazon hasn't?