postfix

Paul Lussier p.lussier at comcast.net
Wed Aug 29 10:02:04 EDT 2007


"Ben Scott" <dragonhawk at gmail.com> writes:

>   Technically speaking, an LDA (Local Delivery Agent, a subcase of MDA
> (Mail Delivery Agent)) would be something like procmail or maildrop.

Or /bin/mail...  Technically it is the LDA which does the
user-existence verification checks, not the MDA.  The MDA simply gets
mail from one hop to the next.  It may be the case that you have
sendmail or postfix be that LDA also, in which case, you must also
configure each properly to inform it where to obtain local mailboxes.

For sendmail, this is the sendmail.cf O operator (among many others I'm sure:)

  # lookup type to find information about local mailboxes
  #O MailboxDatabase=pw

Postfix by default, I believe, delivers mail locally for all users for
which it recieves mail unless specified otherwise.  From the default
postfix main.cf (under OpenBSD anyway):

    # The local machine is always the final destination for mail
    # addressed to user@[the.net.work.address] of an interface that
    # the mail system receives mail on (see the inet_interfaces
    # parameter).

You can then set things like $mydestination, specify local transport
maps, recipient relay maps, etc.  Regardless, you need to specify
*somewhere* who your users are, and AFAIK, there is no way to use
these (ML)DAs to "send a message to all users".  The systems merely
validate an address handed to it, they do on-the-fly mail list
generation.

If Exchange does this, it's because someone thought to build an AD
query into the interface that does so.

> POP and IMAP servers would probably be considered MUAs (Mail User
> Agents) as far as the standards are concerned, since they're all about
> accessing user mailboxes.  Or maybe they've defined another term for
> those that I'm not aware of yet.

I wouldn't consider POP or IMAP an MUA.  An MUA is the end-point
application which recieves/displays/manipulates e-mail.  POP and IMAP
simply store it until picked up by the authenticated user.  Perhaps
'Mail Storage Agent' ?

> Exchange itself will never be an MDA for a Sendmail or Postfix
> system, since you're going to have to do an SMTP relay to get the mail
> into the Exchange system.  So the MDA would be the SMTP relay function
> (the "smtp" mailer for Sendmail).

Never say never.  Let's all agree that we *hope* we never need to use
Exchange as an MDA for a postfix or sendmail system!  Sadly, I just
reviewed my mail architecture.  I had handed it off a year and a half
ago, and it was rather simple and straightforward at the time.  I just
re-inherited it.  It's no longer simple or straightforward.  In fact,
we discovered the theoretical possibility of a mail message doing
figure-8s between our sendmail/cyrus server and the Exchange server!

> That's not really accurate or fair.  Indeed, for most people, the
> distribution ("Red Hat Linux", "Debian GNU/Linux", etc.) is the
> "product", and provides far more functionality than Exchange ever
> will.

I disagree with that statement, but will let it pass for the time
being :)

> One of the reasons I like Linux so much is that everything is
> included "in the box"; I don't need to buy eighteen more tools for
> every server.

Exchange lovers say much the same thing :)

> Even ignoring that, Active Directory and MS-DNS are part of Windows
> (a separate product, purchase, and license), not Exchange.  And the
> SMTP ideas of "MTA", "MDA", and "MUA" don't really apply to Exchange.

Maybe not, however, it's rare that you'll find a shop running Exchange
which does not also run MS-DNS and AD.  They may well be add-on
products, and you may well have to buy them, but from what I've seen,
people who run Exchange just accept that and go ahead.

Running Exchange without them is like running a mailsystem and
internet domain without a DNS server.  Sure, it can be done, but it
seldom is.

> Basically, once the SMTP service receives the mail into
> Exchange, it gets converted into something that smells a lot like
> X.400, and all the SMTP concepts go right out the window.

Let's just paraphrase this to:

 Basically, once the SMTP service receives the mail into Exchange
 all bets are off! 

:)

-- 
Seeya,
Paul


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