Unable to send mail from a form with code

Hello, 

I'm trying to send a mail from a form that contains a code.

I noticed that when my code contains the following lines:

***********

public void InternalStartup()

        {

            EventManager.XmlEvents["/my:myFields/my:group1/my:group2"].Changed += new XmlChangedEventHandler(group2_Changed);

            EventManager.FormEvents.Submit += new SubmitEventHandler(FormEvents_Submit);

        }

***********

E-mail message is not sent and can be found in the log  error message that says:

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Nintex.Workflow.NWNotificationException: . ---> System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated     at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)     at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)     at System.Net.Mail.SmtpClient.Send(MailMessage message)     at Nintex.Workflow.HumanApproval.Message.SendMessage(SmtpClient client, MailMessage msg)     at Nintex.Workflow.HumanApproval.Message.SendEmail(String subject, User user, NWWorkflowContext context, NWContextDataCollection ctxData, MailAddressCollection toAddresses)

************************

Would appreciate help

GIl