Salesforce Error "Insufficient access rights on Cross Reference Id" on Attempts to Send Emails

A user must verify their email address in order to send out emails.  The Salesforce admin must temporarily change the user's email address, then input the user's main or work email address in order to trigger the verification email. 

To verify the user's email address, perform the following:
 

1. Click the gear icon and select Setup
2. In the Quick Find box, search for Users
3. Select Users under the Manage Users settings.
4. Click Edit next to the affected user's name.
5. Change the email address on the 'Email' field to a user's personal email address temporarily.
6. Save.
7. Edit the same 'Email' field with the correct or preferred email address to trigger the verification link.
8. Then save.
9. Check with the affected user's mailbox to verify their email address.