Here is a data point; take it for what it's worth.

I run my own email service (Postfix) on 4 different domains. TLS is properly configured on all of my mailhosts, using certificates issued by StartCom. My servers routinely receive mail from Google, Apple, Yahoo, GNU, and other major email providers. Most of the messages are from various mailing lists.

I occasionally peruse the mail logs, and in the last 3 years, at least, I have never seen an unencrypted SMTP connection. I'm not saying it never happens, I've just never seen it. The most common protocol is TLSv1 with a variant of AES (nearly always 256-bit). Apple's listservs use TLSv1 with 128-bit RC4-MD5, but they're the exception.