"Chris Haynes" <chris(_at_)harvington(_dot_)org(_dot_)uk> writes:
In simple cases, that need only involve a single look-up. The multiple
look-ups
usually arise if you have a Mail From domain who uses a different domain's
servers to send its mail. Almost all small/medium business and 'vanity'
domains
are in this situation. They use an ISPs outbound mail servers.
This has often been stated, and I have wondered why. I am the owner of
such a "vanity" domain and I work for a small (10 employee)
business. Yet both my personal domain and the business domain run
their own mail servers (which are connected via DSL), both for sending
and receiving mail. Doing this is so much more convenient than using
the ISP's servers. For example with incoming mail it is possible to
apply whatever checks (SPF, Antivirus, DomainKeys, RBL etc) the domain
owner wishes to apply so that mail which fails these checks can be
rejected at SMTP time rather then being accepted and a bounce
generated later (which most people agree is not a very good
idea). With outgoing mail, the administrator can easily see if mail
has been delayed and queued, and also can check that (and when) the
receiving MX accepted the mail. When sending using the ISP's servers
it is common to not get a notification of problems sending until the
ISP sends a "Unable to deliver for the last xxxxx" message to sender
after having tried for some time to deliver the message.
So it seems to me that, for systems which have 'always on' connections
to the internet, the benefits of a domain running its own mail server
far outweigh the convenience of letting the ISP do all the work.