SUMMARY: Is Sendmail needed? How to configure mailx? MUA & MTA

Thank you so much for your replies:
"Erwin Fritz" <efritz@xxxxxxxx>
"Lars Hecking" <lhecking@xxxxxxx>
"Andy Malato" <andym@xxxxxxxxxxxx>
"Bill R. Williams" <brw@xxxxxxxx>
"Steven Haywood" <steven.haywood@xxxxxxxxxxxxxxxxxx>
"Darren Dunham" <ddunham@xxxxxxxx>
"Rich Bishop" <rjb38@xxxxxxxxxx>
"Anthony Talltree" <aad@xxxxxxxxx>

Please allow me to use Bill Williams?s excellent
explanation as the summary of this question. I will
supplement the summary if more constructive info is
received.

(I haven?t written a summary yet for the question of
?sendmail ignores DNS MX record? because the problem
is not solved yet).

Bill?s answer:
Whether you need sendmail daemon running to send email
depends upon the version of sendmail.
You are running Solaris 2.6, and mailx works fine.
With the version of sendmail you have installed, you
don't need the daemon running. However, should you go
to a new Solaris and/or sendmail version you will see
a change to this behavior.
For your future consideration...
We have Solaris 5.9 and $Id: sendmail.h,v 8.919.2.17
On this version (and newer Linux distributions) the
security additions and enhancements change the way the
sendmail service works with the mail[x] client. There
is a "client" daemon running in addition to the server
daemon. On Solaris 5.9, the 'ps' shows this:

root 15030     1  0   Jan 13 ?        0:00
/usr/lib/sendmail -bd -q15m
smmsp 15028     1  0   Jan 13 ?        0:00
/usr/lib/sendmail -Ac -q15m

The one running with '-Ac' is the client-level daemon.
Without it the 'mailx' will not work, because the
outbound mail (from mailx) gets staged/queued into a
client queue instead of being delivered immediately by
the mailx command.
A complete description of the newer sendmail is beyond
my ability to explain; however, you can set the
sendmail configuration so that it accepts ONLY from
the 'localhost' which means "only this machine" --
which is pretty much the behavior you have now.
('localhost' only may be the default.) You can also
setup to RELAY only for systems based upon some
criteria you specify: domain.name, IP Address, etc.
You get quite a bit of flexibility and control with
the new version, but you will have to run the client
daemon to send mail.

/////////////////////
Original Question:
I have some questions about Solaris mail here. Here
are our system policies:
1) All SUN systems are mail clients. The mail server
is MS Exchange.
2) All SUN systems are disallowed to receive emails.
3) All SUN systems need to send emails out among our
LANs (not to the Internet).
On a test system running Solaris 2.6, after I stopped
sendmail daemon, I can still send mail out by using
mailx. I understand some MUA can conduct basic MTA
functions.
My questions:
1) Is sendmail needed to run in such a environment?(It
seems not)
2) As a MUA, how does mailx conduct the MTA functions?
For example, how does mailx recognize DNS without
using MX? Its configuration file /etc/mail/mailx.rc
does not contain much information.


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