Apple mail app TLS deficiencies revisitedFri 02 October 2015 by Fred Clift
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.