Upgrading from 14.04 LTS, install hung on install exim4-daemon-light. Had to finally kill the init.d start process to allow the upgrade to continue.
ProblemType: Package
DistroRelease: Ubuntu 15.10
Package: exim4-daemon-light 4.86-3ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-30.35-generic 4.2.8-ckt3
Uname: Linux 4.2.0-30-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Fri Feb 26 20:48:45 2016
DuplicateSignature: package:exim4-daemon-light:4.86-3ubuntu1:subprocess installed post-installation script returned error exit status 137
ErrorMessage: subprocess installed post-installation script returned error exit status 137
InstallationDate: Installed on 2010-01-23 (2225 days ago)
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release amd64 (20091027)
RelatedPackageVersions:
dpkg 1.18.2ubuntu5.1
apt 1.0.10.2ubuntu1
SourcePackage: exim4
Title: package exim4-daemon-light 4.86-3ubuntu1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 137
UpgradeStatus: Upgraded to wily on 2016-02-27 (0 days ago)
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.