Comment 2 for bug 1550646

Just Tested Xenial -> Yakkety with running exim4 - working fine.
Now going for Trusty -> Xenial with running exim4 (and light daemon as MTA) - working just fine as well.

The preinst/postinst scripts use "invoke-rc.d exim4 restart" or "invoke-rc.d exim4 start" a few times - the reported issue suggests it is blocking on those.

That said I'd expect it works on Xenial and later now which I'd recommend you to upgrade to.
If you have had any kind of special exim4 setup that might be part of the reason why the restart/start might have failed I'd ask you to share them so one can try to recreate.