From the sendmail cf/README:

nullclient	This is a special case -- it creates a configuration file
		containing nothing but support for forwarding all mail to a
		central hub via a local SMTP-based network.  The argument
		is the name of that hub.
		The only other feature that should be used in conjunction
		with this one is FEATURE(`nocanonify').  No mailers
		should be defined.  No aliasing or forwarding is done.

However, you do not have to run sendmail¹ on a server unless it is your SMTP server. Sendmail has FEATURE(`nullclient’) for that. Qmail has something similar. But there is nothing simplest than this alternative:

Enter nullmailer:

This is nullmailer, a sendmail/qmail/etc replacement MTA for hosts which relay to a fixed set of smart relays. It is designed to be simple to configure, secure, and easily extendable.

Nullmailer is very easy to configure compared to any of the well known F/OSS SMTP servers. It is not an SMTP server. It is a /usr/lib/sendmail² replacement for your servers or unix shell machines.

Installing nullmailer on a Debian system simply means:

# apt-get install nullmailer
# dpkg-reconfigure nullmailer

and you are all set³. If you are on another system the worst case senario requires that you compile it by hand. However, this is not a difficult task even for the inexpirienced SysAdmin, since the code is compact and easy to read and the directions on how to compile / install fairly simple to follow.

[1] Or postfix, or exim, or qmail
[2] Yes, there was a time that the sendmail binary lived in /usr/lib
[3] Usually this is followed by a dpkg –purge exim4-base exim4-config

Mulberry, (or Thunderbird,or any other GUI email client) needs an SMTP (“outgoing”) mail server which it uses to send your emails. Normally this is set to what your ISP (or administrator) has told you to be. However, sometimes your outgoing email server maybe unavailable¹ (which is furstrating, since most people expect email to be delivered instantly, or at least leave their system² instantly). So why not run a mail queue on your own and let your desktop route your email through the outgoing email server when it becomes available again? Here is a small sendmail.mc that should work with your desktop unix machine³:

VERSIONID(`2006/08/23/00')dnl
OSTYPE(`debian')dnl
DOMAIN(`debian-mta')dnl

define(`SMART_HOST', `[outgoing.example.com]')dnl
define(`confCW_FILE', `-o /etc/mail/local-host-names')dnl
FEATURE(`use_cw_file')dnl

FEATURE(`no_default_msa')dnl
DAEMON_OPTIONS(`Name=MTA-v4, Addr=127.0.0.1, Port=smtp')dnl
DAEMON_OPTIONS(`Name=MSP-v4, Addr=127.0.0.1, Port=submission')dnl

MAILER(`local')dnl
MAILER(`smtp')dnl

LOCAL_RULE_0
# LHS is separated from RHS with tabs, not whitespaces
R$- <@ $=w . >          john.doe < @ example.com. >

LOCAL_RULE_0 makes sure that any email sent from this machine is sent as john.doe@example.com. If this is not expected behavior delete LOCAL_RULE_0 or use FEATURE(nullclient).

[1] Unavailable does not mean neccessarily down. It may mean that some kind of rate-limiting is in place.
[2] Where system for such users is only their MUA (Mail User Agent).
[3] The example is Debian/Ubuntu-centric, but can be adapted to any system that runs sendmail