There are many reasons why emails sent from the system might fail. The reason for a given failure is usually well documented in the rejection email received in the “failure” email sent to the sending address. Almost all mail servers automatically send a rejection email back to the sender with details on the reason and a “failure” status code.
The most common reason an email is rejected is that the sending mail server is judged by the receiving email server not to have the “rights” to send email from your domain. This is a common check that mail servers do on incoming emails to prevent unauthorised use of an email address by spammers.
The receiving mail server checks the Domain Name Service (DNS) details to see which mail servers are “approved” to send email for the domain of the sending email address. If the sending email server (SMTP server) is not “listed” then many mail servers will reject the email. The Sender Policy Framework (SPF) approach is used to “authenticate” a mail server.
Please review DNS SPF settings for email – Help for Online Advantage for more details.
Please also review the details on this post to include a DNS DMARC settings for email – Help for Online Advantage