Re:Ask HN: Email hosting with custom domain support?
by dangrossman on Sunday December 04, 2016 @11:27PM
(#13103358)
Rackspace Mail is probably the leader in this space, with over 4 million paying SMB users. It meets all your requirements, including a 100% uptime SLA and 24/7/365 live support.
https://www.rackspace.com/en-us/email-hosting/webmail