Nintex Workflow Emails for SharePoint 2016 [On-Prem]

Overview: 

Basically, we have a SharePoint 2016 [On-Prem] environment, and we are setting up the messaging & notification for Nintex Workflows in this environment. Find the screenshot below for the details. We are connecting to the`smtp.office365.com` SMTP server.

Issue:
When running a test workflow to test the email, it is not being sent or received. Furthermore, we found the below log in ULS. How can this be resolved? 

Failing sending email: LocalErrorInProcessing.: System.Net.Mail.SmtpException: Error in processing. The server response was: 5.7.3 STARTTLS is required to send mail [AM0PR10CA0114.EURPRD10.PROD.OUTLOOK.COM 2024-03-07T15:34:42.730Z 08DC3D4022D05469]    
 at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)    
 at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, MailAddress from, Boolean allowUnicode)    
 at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)    
 at System.Net.Mail.SmtpClient.Send(MailMessage message)    
 at Nintex.Workflow.HumanApproval.Message.SendMessage(SmtpClient client, MailMessage msg) (Build:4320)

Other information:
We have tried sending an email using a PowerShell command with the same credentials and specifying the port (587) which resulted in success. 

$msolcred = get-credential

Send-MailMessage -From email@email.com -To email@email.com -Subject "Test Email" -Body "Test Text" -SmtpServer smtp.office365.com -Credential $msolcred -UseSsl -Port 587

A similar article on the topic:
Changing the email Port for Nintex Workflow emails.
 

Resolution: 

In the Web.config for our Web App we added the below entry.

<system.net>
    <mailSettings>
      <smtp>
        <network port="587" enableSsl="true" />
      </smtp>
    </mailSettings>
  </system.net>