Emails blacklisted

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Over the weekend we moved one of my clients’ websites off of Hostgator. The move went fairly smooth but the one nagging problem we have is that any emails from her website to the AT&T network are being kicked back with the following message:


SMTP error from remote mail server after MAIL FROM:< > SIZE=1923:
521-xx.xx.xx.xx blocked by xxx
521 DNSRBL: Blocked for abuse.

The hosting company has been working for nearly a week to resolve the problem but isn’t getting anywhere with AT&T. They have tried changing servers, changing IP addresses, and getting a new block of IPs and switching to that. The emails keep bouncing back. AT&T supposedly won’t tell them what the problem is. I’ve used mxtoolbox to check if any of the domain names or ip addresses are blacklisted and they all come up clean. I’ve read through a lot of posts on similar topics but thought I’d take a shot at getting suggestions here.

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