Oh, it's absolutely monitorable (a word?), but there's a huge jump in the ease in which IT can simply read your Exchange mailbox and them setting up effective packet sniffing to read what's going over your particular connection.
This.
Looking through your work email doesn't require much more than getting into the mailbox and seeing what you have in there.
Your personal? They'd have to dump your traffic from every day to try to get to something meaningful. Way too much work.
The long short of it is, if you work disallows you access to your personal email, then you simply don't use it. Use your smartphone or tablet on your breaks, or at your desk if you're allowed.
My wife's office just recently implemented a no personal email policy at work. This means I no longer chat with her throughout the day, but from time to time, she'll pick up her smart phone and we'll chat back and forth through that.
My personal view, as an admin, is that access to personal email at work isn't a huge deal so long as the person does not let it affect their work. We have individuals here that do let it affect their work, so they get further restricted based on their supervisor's requirements.
From an employee standpoint, work can be frustrating. A mental break every now and again helps. For my wife, that was and, to a lesser extent now, is chat. She'd likely do a better job at work if she wasn't completely stressed. Granted, some employers don't see it that way, and that is their call. But when it comes to work email, that is the one thing no one should use for personal use. Use it for work.