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 email@example.com. If this is not expected behavior delete LOCAL_RULE_0 or use FEATURE(nullclient).
 Unavailable does not mean neccessarily down. It may mean that some kind of rate-limiting is in place.
 Where system for such users is only their MUA (Mail User Agent).
 The example is Debian/Ubuntu-centric, but can be adapted to any system that runs sendmail