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Turns out its all about the server not properly authenticating, example here:

Quote:

Sender Verification
Some mail servers now attempt to confirm the sender of a given piece of mail by verifying that the From: address provided is a real one. For example, if you send a piece of mail claiming to be "From: bob@someisp.com", and you send that mail to "sam@someotherisp.com", the mail servers at "someotherisp.com", may contact the servers at "someisp.com", to confirm that "bob" really is a valid mailbox there. If the return address proves to be false, the mail may not be accepted by the "someotherisp.com" servers.

Usually in the case of sender verification, the bounced mail will indicate that the verification attempt failed. The only solution is to use your real working return address on mail you send to this more-selective mail server. For example, if you sent mail out with the "From:" address of "MegaSalesbob" or "REMOVEbob" instead of your real mailbox name of just "bob", that could cause sender verification to fail. Always use your real return address in e-mails if you want them delivered.

Because some mail systems accept all mail, including mail that is for non-existent mail boxes, sender verification systems will usually test to see if the other mail server returns accurate answers. It does this by starting to send mail to a mailbox address on the claimed source system that is extremely unlikely to exist. It then looks to see if that mail server rejects the made-up address or not. If the made-up address is rejected, then the original mail address is tested to see if it is accepted or rejected.

When a Sender Verification attempt fails, the message is usually never accepted by the destination system. It is rejected with messages like these:

550 Sender verify failed

550 sender verify rejected exim

550 verify validity

550 administrative prohibition "mailserver verify address"

550 message to verify they are valid

550 Sender verify failed - upgrade your firewall firmware or settings

550 Not sent - no reverse lookup

550 The destination mail system rejected your return address

550 Sender must exist

550 You must use a working From: address in your e-mail.

550 Domain not found

550 Sender-Rejected mdaemon

550 Sender address rejected: Blocked

550 Sending address not accepted due to spam filter

550 No Such User Here Sender Verify Failed

Note that some systems now store and check a list of sender addresses with a past history of spamming or other e-mail abuse, and reject their mail without actually performing sender verification.

Anyone know how to properly set it up?