This error means the user you are trying to send an email to has a different email address than the one you are using. You will also get this error if their email address is currently inactive.
You may want to check with the person and verify their email address. You can search the address book within webmail, or do a “People” search from the portal.
*Note: This message occurs on accounts with S/MIME enabled. Turning off S/MIME will not fix the error.
S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. Enabling S/MIME on your account allows you to digitally sign emails. You can enable S/MIME on your account by going to the ‘My Account’ drop down menu, to ‘Mail Preferences,’ to ‘Mail Options.’