If you are unable to send email, the two most common reasons are the following:
If you see the following alert
"Microsoft LDAP Directory–No Such object. Possibly, your specified Search Base is invalid."
one of the email addresses in the To, Cc, or Bcc fields is not in the correct format. Check for a missing @ symbol. You may need to check the contacts list for an incorrectly formatted email address.
If you send an email and receive and "undeliverable" message from "postmaster@UDel.EDu," and you are certain that the email address you used is valid, check to see whether the email address listed in the delivery failure matches the email address of your intended recipient.
If the email address doesn’t match, you may have a corrupt automatic completion cache entry in Outlook. Follow these steps from Microsoft to correct the entry: