In November, Google indicated that they're working on a warning indicator to be shown inside of Gmail, indicating when an email message was transmitted over an unencrypted connection.
What does it mean when an email message was sent without using an encrypted connection? It means that your MTA (mail transfer agent-- aka a server sending email messages) wasn't using TLS (Transport Layer Security) when communicating over SMTP (Simple Mail Transport Protocol).
It doesn't mean your message was intercepted by bad guys. And mail servers had been communicating with each other in an un-encrypted fashion for many years. But in this modern age, where individuals (and even big companies) have begun to raise concerns over who has access to an individual's online information, Gmail is taking a stand to say, we want everybody to encrypt email delivery connections so prevent (or at least reduce the chances of) those messages from being intercepted or copied in transit.
If you work for an email service provider or email platform, you know that any time a major ISP or webmail platform puts up an alert over a certain type of message or scenario, your customers are going to be concerned. Even if this notice doesn't really indicate a failure condition, they're going to be worried that it does suggest that to their end subscribers. In short, they won't want this alert to show up for emails they send.
It's quite a savvy move on Google's part. I am sure that email platforms and email service providers that don't already universally support TLS are figuring out how quickly they can implement it.
Does TLS improve email deliverability? No, not directly. But, consider this: if mail delivered without using TLS is going to throw up a big red flag in the Gmail user interface, that's going to cause an issue with subscriber confidence. Calls to customer services. Incorrect assumptions that a message isn't safe or might be fraudulent. These are not concerns you want associated with your email marketing program.