Fix held broken packages on debian?

"... But it is not going to be installed" generally means that a serious dependency conflict will ensue if it's allowed to go on.

Try the following command:

aptitude why-not citadel-mta

why-not basically checks dependencies and returns the reasons it would have to not fill a particular dependency automatically.

In the case of my system at home:

shadur@leviathan:~$ aptitude why-not citadel-mta
i   exim4-daemon-light Conflicts mail-transport-agent
p   citadel-mta        Provides  mail-transport-agent

Apparently citadel-mta is a full-on MTA and will therefore replace whatever mail-transport-agent package you currently have installed. Explicitly telling it to install citadel-mta as well should be enough to break the deadlock.

NOTE: Doing so means your current mail server software will be replaced by the one that comes with citadel. Make very sure that that's what you want before you do this.

I found that installing with aptitude rather than synaptic or apt-get ended up working. aptitude is smarter and offered me resolution to the problem.

Edit: I did have to skip the first resolution suggested to get one that actually worked.