A quick update:
Mac OS X El Capitan has hit the streets so I retested. The mail app still can not make any TLS connections using TLSv1.1 or 1.2.
Also a /u/Tulsagrammer on Reddit pointed out to me that a the mystery of why this is so is even greater: For (outbound) SMTP connections, the mail app can and does speak TLSv1.2. It is ONLY imap connections that are limited to TLSv1.0.
Frustration with Apple mail app on IOS and Yosemite
At work I recently have been irritated by a problem that was exposed with Apple's mail app, both on IOS and Yosemite. Among other things, I maintain an imap server (using dovecot ) for our office email.
First some background
Dovecot makes it easy to enable TLS, and disallow unencrypted ...read more