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    AT&T & Yahoo - Your customers are not receiving their email - 5.3.0 flph384 DNSBL RBL 521

    Thanks Popowich. That's interesting. My sender is on a private IP which is not listed with any of blacklists. I'll keep an eye out for this. Thanks.
  2. C

    AT&T & Yahoo - Your customers are not receiving their email - 5.3.0 flph384 DNSBL RBL 521

    Take a look at AT&T's error message page. AT&T The bounce back containing 521 indicates 'The sending system has been identified as a source of spam.' Thus you got too many spam complaints or your sending I.P. may be blacklisted.
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