My guess is about 50% use hosted emails. That is a good question. Securely changing passwords has been the main issue for me or getting a new password to someone who has forgotten one. That process can take several hours meaning employees go without email for that period of time.
Ideally, setting new passwords for email account and changing lost ones should be able to be done by the email user only, like on BC. The one time initial setup, entering user name and recovery email address should be able to be done by Treepl partner or website client with correct user permissions.