Unable to send an email via a PHP/ASP.NET script in Plesk for Windows Server: 503 This mail server requires authentication when attempting to send to a non-local e-mail address

Symptoms

When sending an email via a PHP script on Windows, the operation fails and the following error message is logged in the MailEnable Activity log (MailEnable > Server > localhost > Services and Connectors > SMTP > Logs > Activity):

503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.

Cause

The SMTP server requires authentication in order to send mail to non-local addresses.

Resolution

1. Log in to Plesk.

2. Go to Tools & Settings > Mail Server Settings > White List and click Add Network.

3. Add localhost addresses:

   127.0.0.1/32, ::1/128

   Note: If localhost addresses are added to the White List, emails will not be counted towards the Outgoing Mail Limit.

4. Click OK.