On Aug 11, 2010, at 3:01 PM, Tony Finch wrote:
On Wed, 11 Aug 2010, Arnt Gulbrandsen wrote:
Some SMTP servers (even some smarthosts) will act as an SMTP client before
replying to the dot. That was true twenty years ago (when random unix boxes
acted as a general spool and MUAs could run e.g. sendmail -bs or smail
-iforget to talk to an SMTP server), and it's true now.
I think that's only true if you are running sendmail -odi (deliver
interactively option). Smail does local delivery in the foreground and
remote delivery in the background (dunno if that includes DNS lookups).
Exim can't be told to do foreground deliveries while talking to an SMTP
client, and qmail and postfix don't have the right architecture to allow
This sort of supports my earlier comment about the design of mailers
changing in this respect about 15 years ago.
Spam filtering appliances and services sometimes run as, effectively,
proxies rather than store-and-forward MTAs. Postini works in this
way, I think, as do some of the enterprise filtering appliances that
people protect their Exchange servers with.
If anything, they're a more recent approach to mail filtering than
their competitors that are cobbled together using the store-and-forward
MTAs you mention. They have significant deployment advantages
and some disadvantages.