Why am I getting email bounces that say "451: Could not complete sender verification"?
It isn't us, it's the server at the other end. Before accepting email from outside servers, we verify
that the sending address is reachable. We do this in order to:
Make sure mailservers talking to us conform to the internet RFC's (published standards).
Verify that if we need to send a bounce message back to the sender (as opposed to email getting
blackholed with everyone wondering what happened to it).
Block spammers and viruses that send email with forged From: addresses (e.g. "firstname.lastname@example.org")
We can disable this check for your domain, but two things will happen: Email
for your domain will be slightly less reliable (since we can't verify whether
bounces or replies will work), and you will start getting orders of magnitude
more spam and viruses than you do currently.
We recommend talking to the folks at the mailserver
at the other end (the one trying to send the mail to you).