I checked all proposed solutions in aptitude. I think the reason exim4-daemon-heavy wasn't proposed was that I didn't have the exim4 binary (meta) package, which lists both daemons as dependents. exim4-base just has a breaks: versioned dependency on both daemons, but nothing to pull them in. That's probably appropriate since the daemons depend on it.
If that analysis is correct, then things get messy with the exim4 binary package installed. exim4-daemon-light is pinned, but not exim4-daemon-heavy, though I presume both had the same problem. So if aptitude proposed -heavy as an alternative (not sure if it would), I'm not sure what would have, or even what should have, happened next. Applying the pin to the source package seems safer, though I don't know if that's possible. Ross
