Symptoms

The following error keeps on recurring in the K2 HostServer Logs and a Default Email Event cannot be sent:

"Error","General","0","ErrorMessage","SourceCode.Workflow.Common.SmtpMail","0 SMTP mail send failure: Verify the SMTP connection strings are correct and that SMTP is working properly. Exception: Mailbox unavailable. The server response was: 5.7.1 Unable to relay"
 

Diagnoses

We have checked the ConnectionStringEditor.exe several times, we have checked the "From Address" variable in the Environment Library, we have deleted and recreated both, we've even hard-coded the email address of the "From" and "To" fields in the workflow default email event.
 

Resolution

It seems as though the mailbox we want to send the email to doesn't exist. The error message that K2 produces is a bit misleading and I've linked this to a feature request to improve the logging of this error.

If you use SMTP to send emails and encounter this error, please make sure that the mailbox you send your emails to actually exist. If this is a case of the Identity Service not having updated the user's current (or changed) E-Mail address, then you'll have to run the Force Identity Refresh Tool on that specific user to update his/her mailbox.