Force-quit both the iAnnotate and Mail apps, then reboot your iPad. Upon restarting, email a document to yourself and see if it succeeds.
- Double-click the iPad Home button.
- Swipe left or right to locate the app you want to close.
- Swipe the app up to force-quit it.
Try Resetting Mail Accounts
If the above procedure doesn't help, or if you receive notifications about an email error, you may need to open the iPad Settings app and reconfigure your email accounts.
In Settings, select the "Mail, Contacts, Calendars" menu. Scroll down to "Default Account" to identify which account you have set as your default.
Scroll back up to "Accounts," select the account you currently have set as the default and tap "Delete Account."
Use the "Add Account" button to re-add this address and reset it as the default.