Is it just me or are all of the features they list on the landing page exactly what Google inbox did before they stupidly shut it down? I loved the inbox UI...
Superhuman has read receipts. AFAIK no email clients offers that outside of email marketing software.
I actually consider read receipts unethical. It enables stalkers, and enables bad employers to make judgements about their employees' personal time and sleeping habits. Good e-mail clients should never load any external resource without the user's explicit consent. It should be enforced that images to be displayed in the e-mail should be attached to the e-mail itself.
AOL had this a very long time ago, but I think it only worked with other AOL accounts.