Issue
K2 Workflow Instances go into error state on the Send Email step.
Error Code
Looking at the K2 HostServer log files you will see the below error being logged:
------------------------------------------------
SourceCode.Workflow.Shapes.Execution.Mail.EWS: Exchange error code: ErrorInvalidOperation Exchange error: Microsoft.Exchange.WebServices.Data.ServiceResponseException: SendOnly cannot be used by a user without a mailbox. Use SendAndSaveCopy and specify a folder ID in a mailbox to send an item from an account that doesn't have a mailbox.
------------------------------------------------
Resolution
You should try the below steps which might resolve this issue:
1) Make a backup of the "EWS URL" value in the ConnectionString Editor Tool in K2.
2) Make a backup of the "Mail Server" Environment Field value in the K2 Management site.
3) Change the two values above and save them. Then restart the K2 Server Service.
4) Change the values again back to their original values and restart the K2 Service Account.
Additional Information
You can try to test if the Exchange Email configuration is working from outside the K2 Product Stack? An Exchange connection Test App can be used for this.
You will see the same error presented in the Exchange Test App which confirms this error message is being returned from Exchange side and not from K2.