This error could mean that you have the senders domain in your hosting/reseller account and hence its failing the spoofing check.
If the customer is sending from an external mail server and mail service is not hosted on our servers ensure the mail service for the domain is disabled within Plesk or cPanel.
551 This mail server requires authentication before sending mail from a locally hosted domain
Related
Domain example.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed503 valid RCPT command must precede DATAEmail Bounceback: Please add more content, cut down on HTML links, use fewer naughty words etc. Also, ask your IT dept to make sure your mailserver has REVERSEDNS, SPF, DKIM, and is not on any black lists.When I send out emails they land in the spam or junk folder on the receiver's side?Why Is My Email Account Blocked and How Do I Unblock It?
Still stuck? Open a ticket.