Recently, I had a K2 process sending notifications when escalations occur. This was done by sending an email to a gateway. During testing I started encountering email relay issues. The error log on my process portal stated that “Mailbox unavailable. The server response was: 5.7.1 Unable to relay for…”
A quick visit to my SharePoint central admin page revealed that they were using a different SMTP server to mine. I used an SMTP server picked up by the blackpoint installer.
The email relay error usually comes to light when you start sending emails to users outside your domain. This behavior is a good thing. So, how do you work with this issue? You need to have permissions for relaying. Sort that out with the sys admins. Then use the correct SMTP server for relaying from a valid from email address.