If you reach a dead end using APT you can download package files from Debian mirrors and install them using dpkg. If you do have not have access to the network you can look for cached copies of package files in /var/cache/apt/archives/.
# dpkg -i fetchmail_6.2.5-4_i386.deb
If attempting to install a package this way fails due to dependency violations and you really need to install the package then you can override dependency checks using dpkg’s –ignore-depends, –force-depends and other options. See dpkg(8) for details.