Symptoms

We have a workflow that is failing due to the following error:
The email address, Null or Empty, does not meet the requirements.

However, we are sure that we have filled out the To:, CC: and BCC: fields.
 

Diagnoses

The issue was due to a trailing Semi Colon on the Email Event. This semi colon caused K2 to not be able to read the email addresses.
 

Resolution

Make sure that none of the To:, CC: or BCC: fields have a trailing semi colon. If they do make sure you delete it and re-deploy the workflow.