My ISP (Optimum Online) recently started bouncing emails from my web host provider (racknine.com). The message I'm getting is (I've inserted *'s where my last name had been. All email addresses are valid). This has worked without problem for months. Any idea what could have caused this to fail? Getting any tech answer out of a large ISP like optonline seems futile. :-(
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This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
*********@optonline.net
(ultimately generated from ******@*******.com)
SMTP error from remote mailer after MAIL FROM:<*******adm@h10016.racknine.com> SIZE=3361:
host mx1.optonline.net [167.206.5.6]: 550 5.1.8 invalid/host-not-in-DNS return address not allowed
