Most of our mail servers now attempt to confirm that the sender of a given piece of mail is genuine by verifying that the From: address provided is a real one. For example, if you send an email claiming to be "From: email@example.com", and you send that mail to "firstname.lastname@example.org", then the mail servers at "someotherdomain.com", may contact the servers at "somedomain.com", to confirm that "fred" really is a valid mailbox there. If the return address proves to be false, the mail may not be accepted by the "someotherdomain.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 "MegaSalesfred" or "REMOVEfred" instead of your real mailbox name of just "fred", 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
In some cases, you may not be able to receive email because the people sending you the mail are not using real email addresses. In these cases, you will need to ask the sender to use a valid address that can be verified.