Yahoo should probably focus on getting the basics of email deliverability right first. They could have the best client side features by a distance, and I wouldn't go near them and would continue to tell others to avoid using them.

The number of times their servers reject emails at random, causing yahoo email users frustration, is astonishing for an email service provider. Contacting their postmaster to spell out the issue results in a dispute over email headers (which they insist can be obtained from their email client even though the emails are being rejected server side) - and their numerous problems go unsolved. We tell users to switch away from yahoo if they want to receive emails with a reasonable probability.

We've never had issues sending millions of emails to Gmail, outlook, or pretty much any large ESP other than yahoo.