$ sudo apt-get install ssmtp
Reading package lists... Done
Building dependency tree
Reading state information... Done
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:
 ssmtp : Depends: libgnutls-openssl27 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

---------------------------------------------------------------------------

$ sudo apt-get install libgnutls-openssl27
Reading package lists... Done
Building dependency tree
Reading state information... Done
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:
 libgnutls-openssl27 : Depends: libgnutls26 (= 2.12.14-5ubuntu3.4) but 2.12.20-7 is to be installed
E: Unable to correct problems, you have held broken packages.

---------------------------------------------------------------------------

$ sudo apt-get install libgnutls26
Reading package lists... Done
Building dependency tree
Reading state information... Done
libgnutls26 is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.