CAUSE
The senders' SMTP e-mail address that is configured in Outlook and Outlook Express (POP3 or IMAP client) does not match an address in the Exchange directory and cannot be validated by the Internet Mail Service.
RESOLUTION
This resolution assumes the Exchange Administrator wants to retain the anti-spoofing configuration on the Internet Mail Service.
In Outlook Express, ensure the e-mail address entry (from the Tools menu, click Accounts , click the Mail tab, select the User name , click Properties , and then click the General tab) is the same SMTP e-mail address for the Exchange user located in the Exchange directory (under the Exchange user properties, E-mail addresses tab, SMTP address).