2009/11/23 jan gestre :
> I have a Postfix server behind a gateway firewall that does content
> filtering (SpamD on pfSense) and greylisting and we have an
> application server that uses Postfix as its MTA, the problem is after
> the initial 451 response the latter quits and send an Undeliverable
> Mail to sender, what changes needs to be done on the latter in order
> for it to resend email successfully, I don't have a problem with other
> third party mail server except for this one. Since I don't have access
> to the latter I can't provide any logs except for the following:
>
> ---
> This is the Postfix program at host mail.latter.sample.
>
> I'm sorry to have to inform you that the message returned
> below could not be delivered to one or more destinations.
>
> For further assistance, please send mail to
>
> If you do so, please include this problem report. You can
> delete your own text from the message returned below.
>
> The Postfix program
>
> : host webmail.former.sample[210.21x.xx.xx] said:
> 451 Temporary failure, please try again later. (in reply to DATA command)
It sounds like your application server isn't doing the right thing.
The 451 error code indicates a temporary error, and that the sender
should try again later. If it doesn't retry, that is the sender's
fault, it's not following the spec.
The best solution is to fix the sender. An alternative would be to
whitelist the sender on the Postfix box. As you say, you don't have
access, perhaps you can try contacting whoever administers the server.
Greylisting is an anti-spam measure (ie. "incoming" mail). Your mail
sounds like it's outgoing, so it shouldn't be unreasonable to get
whitelisted.