I can confirm. I was able to add a corporate Exchange account to the OS X accounts handler before Mavericks and I assumed it was being pushed to mail.app since the account would show mail, contacts, calendar and tasks sync in the accounts handler. However, I always suspected there was problem with real "pushing" of email.
My work Exchange account email would arrive approx 1 - 5 minutes slower to mail.app than any other service. Email, calendar, contacts,tasks push was near instantaneous to Outlook 2011 on the same MacBook Pro, a PC with Outlook 2010, an iPad and an Android phone.
I believe the mobile devices defaulted to the Active Exchange Sync protocol on the server. I know that OS X doesn't support this but iOS does, so we can rule them out of the equation. You can see this when you add an Outlook.com account to an iOS device. It will push mail, contacts, calendars and tasks in real time if you send yourself a test email. OS X will only fetch mail and notes in the settings options.
I'm guessing there was always a problem with OS X using any Microsoft protocol to retrieve mail but it is FAR more obvious now. I bet Apple set standard corporate Exchange accounts to fetch and it never truly pushed anything. Of course the AES protocol has never been supported by OS X otherwise it would use it!
I can't receive Exchange email at all in Mavericks mail.app unless I manually press the check mail button now. Don't forget the server auto discover problems when on the work LAN vs at home.
I use Windows 7, OS X, Android and iOS devices frequently. I've been looking to commit to a single unified OS/application ecosystem. THE FIRST REQUIREMENT IS THAT THE SYSTEM SUPPORTS MY WORK EXCHANGE PUSH SYNC. I have multiple other accounts like outlook.com, yahoo.com, aol, Facebook, Twitter, LinkedIn, etc. I find OS X is the most intuitive with the account handling baked into the OS. I cannot make the commitment to commit to the Apple ecosystem when the default account handler doesn't support Exchange push.
