Don't pass the '-a' argument to systemctl
Using the '-a' argument makes systemctl display every unit it knows about regardless of whether it's installed or not. There are several ways it can know about services that don't exist. The most common
case where this happens is if a service wants to make sure it is started before/after another service, but doesn't necessarily require it to be running if it's not installed. It defines Before/After dependencies for such services in its systemd unit file, which affects the ordering of services being started, but not whether they're actually started.
So you probably have some service that wants to start before/after exim, but doesn't necessarily require it to be there. (Perhaps because it could work with sendmail or postfix as well). This is nothing to worry about, hence why systemctl hides it from you unless you specifically unearth it with the '-a' argument.
If you want to view all running services and ones systemd tried and failed to start, just use 'systemctl -t service'. If you want to see a list of all services and whether they're enabled, disabled, or statically enabled, use 'systemctl -t service list-unit-files'.
