This error occurs if you’re sending via SMTP. The SMTP server rejects the mail for several reasons:

Your server dosen’t allow different sender and usernames

Some servers require that the From email address is the same as the email address used to authenticate. Check your from name and match it to the username you use to login to your SMTP server.

The file size of your message is to large

Some servers reject a message if the body is too large. HTML emails in general are bigger than normal text messages but you should try to decrease the size by removing some content or disable embedded images.

The receiver is blacklisted or blocked

Sometimes certain email addresses are blocked and to prevent getting blacklisted the server rejects your message. Best resolution is to choose a different delivery method.

It’s a temporary issue

Sometimes the email server responds with this error if some limits have reached or you’re sending “too fast”. Please pause your campaigns for a while and try again later.

If you are sending via Amazon SES you must have verified the from, reply-to and bounce address you are using in your mailings.