You can't disable the pin code if you are using stock email application, but there are alternative email clients that know to bypass this requirement,
try Email+, it may be exactly what you have been looking for:
Email+ is a fully featured E-Mail application based on Gingerbread 2.3.7. While being almost identical to the standard Android Email application, and while maintaining the exact same look and feel you are already used to, it allows you to use your Exchange or ActiveSync accounts (mail, calendar, contacts) without forcing you to enable the annoying pin code or password. https://market.android.com/details?id=com.finik.email
