After your email account was moved to Exchange 2010, if you get bounced email messages and the following error message:
This is due to a bad cached auto complete or bad email addresses entered in the contacts. Usually it only happens in Outlook 2010, you can send email in the Outlook Web APP without any problem.
To fix the problem in Outlook 2010:
Delete the auto complete from the TO field
Delete or re-create the contacts in your address book
Then, type the email address that you know is valid in the TO field or contacts, resend the message.