On a reboot, or a restart of the mail system, I see the message Mailer daemons: exim abandoned: unknown, malformed, or incomplete option -bz sendmail. What does this mean?

-bz is a Sendmail option requesting it to create a `configuration freeze file'. Exim has no such concept and so does not support the option. You probably have a line like

/usr/lib/sendmail -bz

in some start-up script (e.g. /etc/init.d/mail) immedately before

/usr/lib/sendmail -bd -q15m

The first of these lines should be commented out.