Look at what the error message says:
This may mean that the package is missing, has been obsoleted, or
is only available from another source
The package is not available from the source you are using.
Please, give us some more information,. for example:
what your /etc/apt/sources.list file has in it. exim4 on Debian 8 Jessiehttps://wiki.debian.org/Exim
Exim generally comes with default Debian installation. If you need to use ACL and other features you may need to install exim4-daemon-heavy
So some details on how you installed Debian ? It should have been included,
unless it was a very minimal install,..I can not guess at that, please give details.
There is more details here: https://packages.debian.org/jessie/exim4
I know, somebody will probably jump in , and tell me I should not try to answer
this , because I can't,...the main reason is because the OP does not include enough
details, but since no one else seems to be willing to try to help , I figure at least
it may help them to know more details are needed.
Or a little search "foo", on how to install packages on Debian, etc. and the OP might be able to figure it out on their own.
I apologize, for not knowing the exact answer, and for what probably is just another use less post on my part.
If any body else has a better answer, I am sure the OP will appreciate it.