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Unable to correct problems, you have held broken packages

I have Dockerfile that I have used many times without an issue. Now I need to add some packages to it (ssmtp and sendmail) and when I add them the build fails with:

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
sendmail : Depends: sendmail-bin but it is not going to be installed
Depends: sensible-mda but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

If I add those packages to the list I then complains about others. What does this ‘held broken packages’ message mean and how do I fix it?

Here is the first part of my Dockerfile:

FROM debian:jessie
ENV HOME /opt/django/CAPgraph/
RUN echo “deb http://http.debian.net/debian jessie-backports main” >> /etc/apt/sources.list
RUN (apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y build-essential git python python-dev python-setuptools nginx sqlite3 supervisor mysql-server libmysqlclient-dev vim cron unzip software-properties-common python-software-properties openjdk-8-jre xvfb wkhtmltopdf sendmail)

tried adding sendmail-bin and then it failed with:

The following packages have unmet dependencies:
sendmail-bin : Conflicts: mail-transport-agent
ssmtp : Conflicts: mail-transport-agent
E: Unable to correct problems, you have held broken packages.

I then added mail-transport-agent and it failed with:

Package mail-transport-agent is a virtual package provided by:
opensmtpd 5.7.3p2-1~bpo8+1
ssmtp 2.64-8
sendmail-bin 8.14.4-8+deb8u1
qmail-run 2.0.2+nmu1
postfix 2.11.3-1
nullmailer 1:1.13-1+deb8u1
msmtp-mta 1.4.32-2
masqmail 0.2.30-1
lsb-invalid-mta 4.1+Debian13+nmu1
exim4-daemon-light 4.84.2-2+deb8u3
exim4-daemon-heavy 4.84.2-2+deb8u3
esmtp-run 1.2-12
dma 0.9-1
courier-mta 0.73.1-1.6
citadel-mta 8.24-1+b3

E: Package ‘mail-transport-agent’ has no installation candidate