If you are unable to send an email, the two most common reasons are the following:
Incorrect email address
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 fileds is not in the correct format. Check for a missing @ symbol. You may need to check the contact list for an incorrectly formatted email address.
Corrupt automatic completion cache
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 in the Delete one or more suggested recipients in the Auto-Complete List section from this Microsoft Article.