I have a forwarding mailserver inside my office (sendmail) which does some virus check on outgoing mail.
All goes fine except with a particular domain, which always rejects my mails. I spoke with the maintainer of that domain and he says his mailserver is rejecting mails because it is doing greylisting.
But mails are rejected with errors like 5xx which I know are permanent failures.
I assert that his mailserver should return an error like 4xx for greylisting.
He replies that my mailserver is misconfigured and should keep retrying after the first 5xx error.
Who's right and who's wrong?
And, if I'm wrong, can someone point me out on how to configure sendmail to retry sending after the first 5xx error?