AT&T / SMTP error code 550
smtp;550 5.7.1 Connections not accepted from servers without a valid sender domain. ## Fix reverse DNS for x.xx.xx.xx
High severity
Usually this is a reverse DNS (PTR) issue with the sending IP, but sometimes lookup failures by AT&T can cause this bounce to be a false-positive. After confirming reverse DNS is properly set up and the record has fully propagated, try resending the message.
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