We have about 7bn total unique active (3 months) accounts in the world, 50-60% is Gmail, Outlook.com, Yahoo and a few more, 40-50% are real IMAP servers, of which 76% use Dovecot. Very large deployments include companies like Comcast, Deutsche Telekom, 1&1, LGI (Ziggo, Virgin Media, UPC) with 10m+ user deployments, and yes, a veeery long tail of millions of small servers and anything in between. But COI will also play fine with GMail and friends.