[Exim] size restriction

Hello,

It's more a concept question, maybe I'm just blabing about something I don't
understand, but...

I've set
message_size_limit=300K
It works like it shold, longer messages are denied, with a returning message
saying :

... while talking to mail.pbz.hr.:
>>> DATA
<<< 552 Message too large
554 hana@???... Service unavailable

and attaching the original message.

My question is - is my exim mail server receiving the whole message before
saying it's too large or does it stop the transfer after first 300K? I guess
it's receiving it all because othervise it couldn't attach the problematic
original message in the denial message.

Could it be possible to stop this data transfer once the limit is reached?
Otherwise, a sort of DOS attach could be possible on mail servers?

---
Hana Breyer Priselac
system engineer