Symptoms

During the install I tested the SMTP Connection with the "Test" button. I've been able to receive emails previous but for some reason I haven't been able to in the past week or so.

Here is the error I get when the particular workflow mail step is being executed.

"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: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated. 

Diagnoses

The issue was related to the administrator configuration of the mail server. The mail server authentication was not allowing access to the mail server from the k2 server. 

Resolution 

Once the correct settings were used on the mail server, the issue was resolved and mail was able to be sent. Client checked the MailServer Administrator and confirmed that Require SMTP Authentication is unchecked for all 4 options (i.e. Local to local e-mail addresses, Local to external e-mail addresses, External to local e-mail addresses and external to external e-mail addresses).