Re: Recipient validation - WAS: Re: Moderated posts?

[Joe <joe@jretrading.com>]

On Wed, 15 Oct 2014 22:12:41 +0100
Brad Rogers <brad@fineby.me.uk> wrote:

> On Wed, 15 Oct 2014 21:44:30 +0100
> Joe <joe@jretrading.com> wrote:
> 
> Hello Joe,
> 
> >It is *not* OK to silently delete an already accepted email, it does
> 
> Unfortunately, it happens;  Send an email with a large attachment(1)
> and there are quite a few servers that will silently drop it.  The
> worst of it is you can never know for certain if you're going to get
> "bitten" because routing can vary.
> 
> (1) 4Meg or so used to do the trick.  Things might be different now as
> more and more messages contain massive amounts of HTML and imagery.
> 

Possibly so, but in every case somebody has messed up. Firstly an MTA
trying to send a large file should query whether the receiving server
is happy with that. There will be receive limits on email size directly,
but also the recipient's mailbox may not have enough spare space, and
policy may be to refuse the large email rather than quarantine it
somewhere else. If the receiving server isn't happy, the transaction is
aborted and the sender told.

If the sending server sends more than the receiver is happy with, even
if no protest was raised earlier, or no warning of the size was
given, the receiver can simply not acknowledge receipt, or indeed
just terminate the TCP session, and again the sending server notifies
the email sender, possibly after a couple of re-tries if it has not
been explicitly told that the message isn't welcome.

If a large email is accepted, but only later it is found that it cannot
be delivered by reason of some policy, then it's up to the receiving
server to tell the recipient. 

I'm not for a moment doubting that it happens as you say, but there's
no need for it in the case of a legitimate email, it is always possible
either for the receiving server to fail to complete the SMTP
transaction, or for recipient-end processing to inform the recipient of
any post-acceptance delivery problems. Either the sender or the
recipient *can* be told.

-- 
Joe