I created a workflow with a user task. The task was to approve, reject, or rework a form. The workflow would kick in as it should but the notification would fail to send. I opened Blackpearl server and noticed an error 61007. Permanent failure sending message "task": System Exception: No destination to send the message "task". EWSMessageDestination:1 The email address "address" does not have 'send on behalf of' permissions on Exchange Online. I was able to send emails from a workflow when I used an explicit email but the task event notification was failing. The email account is configured for the admin account. I noticed that when sending explicit email in a workflow the sender was the admin account. When sending the event notification the sender was the service account. So the way I resolved this issue was to navigate to the Blackpearl folder under "Program Files (x86)>K2 Blackpearl>Host Server>Bin" I found the K2 Hoster Server.exe file. I did a control find and searched for the service account. What I found looked like this.
<system self="K2Domainservice account" enbleListeners="True" allowAmbiguity="true" />
I changed this to reflect the Admin account.
<system self="K2Domainadmin account" enbleListeners="True" allowAmbiguity="true" />
I restarted Blackpearl server and tested the change. Event notifications were now sending as they should.
Thanks