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.
The SMTP server requires authentication in order to send mail to non-local addresses.
Go to Tools & Settings > Mail Server Settings > White List and click Add Network.
Add localhost addresses:
Note: If localhost addresses are added to the White List, emails will not be counted towards the Outgoing Mail Limit.