You can try Spark developed by our team at Readdle. It works on macOS and iOS, and an Android version comes soon.
We want to build the best email client for business, and we already have millions of users. Spark has most of the features you're talking about. You can snooze emails or schedule them to be sent later, and our smart inbox works with multiple accounts. There are also some cool features for team collaboration: share drafts and links to emails, discuss and create emails with your team.
As for separate read receipts, it's a great idea, and we should definitely think about adding this feature!