Error When Sending Email: 'INSUFFICIENT_ACCESS: insufficient access rights on cross-reference id'

If users in the Free or Trial organization try to send an email from within the Salesforce application after composition on the send, they'll see an error similar to "INSUFFICIENT_ACCESS: insufficient access rights on cross-reference id: 005XXXXXXXXXXXX" containing the valid user id of the user. However, no error message is displayed when sending an email from a case record.

This error can also be seen when a System Administrator uses "login as" for an unverified user in ANY ORGANIZATION and tries to send email from Salesforce as that user.

If you receive this error, you need to verify and authenticate your email address used by following the steps in this article. Salesforce requires email verification to prevent unauthorized email sending and protect org security