Most people assume their emails at work are private. Those people are wrong. There are very few laws that protect you from email snooping at work, and most are pretty worthless and weak. That means you should assume your employer is monitoring your email and act accordingly.
If you get funny emails that are in bad taste, don't forward them. You might be accused of sexual (or racial) harassment. Don't write your lawyer on your work email. And for heaven's sake, don't send love notes to co-workers.
My rule of thumb is this: If you don't want it on the front page of the company newsletter, don't email it at work.