Solution:

(These types of problems don't have good solutions. But only some haking-ish)

You should first try this command in a terminal:

sudo apt-get -f install

If it doesn't work, process with haking-ish solution.

Try temporarily moving the configuration script of ssmtp.

1. Open a terminal and execute this command to move them to the current dir

   
   

     sudo mv /var/lib/dpkg/info/ssmtp* .
   
   

   
   



2. Then try using apt-get

   
   

The cause of the problem:

The previous installation or removal might not completed successfully, leaving some configuration step to be done for ssmtp package. Since, dpkg won't progress without first completing the steps, subsequent use of apt-get now get stuck on that step.