Every mail server requires its users to provide a username and password to send mails. This is to keep spammers out. But very often valid mail users forget to turn on authentication in their mail clients, and the MX rejects their mail.
A quick look at the support tickets shows that 95% of ‘Relay Access Denied’ errors are caused by incorrect SMTP settings.
So, when customers come to us facing this error, the first thing we check is their mail client settings. By guiding users with the correct settings specific to each mail client and mail server, we help them send emails without this error.