Not sure if this will help, but here are the instructions my Exchange hosting company gave me. I too got invalid certificate (addressed in step 7 below), but it worked anyway. You'll have to replace the URL's/settings with your own.
1. From the devices main screen, go to: Settings ->Mail, Contacts, Calendar ->Add Account -> Microsoft Exchange.
2. Under account info, in the Email address box, enter your hosted Exchange email address.
3. Leave the Domain field blank.
4. In the Username box, type in your email address. This needs to be the main email address for your account (i.e. the one you are using when logging into Outlook Web Access).
5. In the Password box, type in your account password.
6. Tap on the Next button at the top of the screen.
7. Your device may fail stating "Unable to Verify certificate." If this is the case, tap Accept.
8. A box labelled Server will appear. Enter: webmail.ihostexchange.net
9. Tap on the Next button at the top of the screen.
10. The device will then verify the server saying "Account has been verified."
11. A new screen will appear asking you to "Select info to synchronize using Exchange." It will display Mail, Contacts and Calendars. Turn the features ON if you desire to have them synchronized with your Exchange account. Once satisfied, select the Save button.
12. You will then be taken back to the default Mail, Contacts and Calender screen. You can adjust the mail settings preferences here at your leisure.