[UA-EAI] Authoritative figures on mailboxes supported by various platforms

Does anyone know of a credible report that shows how many email addresses are under management by a service or being supported by an application? https://techcrunch.com/2016/02/01/gmail-now-has-more-than-1b-monthly-active-... for example shows that Gmail has more than 1b (1.2 b I've seen elsewhere). Don Don Hollander Secretary General - UASG Skype: Don_Hollander