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.
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503 valid RCPT command must precede DATA
When sending email, you receive an immediate bounce such as the following:Subject: RE: XXXXXX -...
Domain example.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed
Are you getting this error returned when sending email? Chances are one of your email accounts...
Email 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.
Our spam filters at times will return the following error: Your message to...
