package:postfix:2.11.0-1ubuntu1
Preparing to unpack .../postfix_3.1.0-3_amd64.deb ...
 * Stopping Postfix Mail Transport Agent postfix        postmulti: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postmulti: fatal: unable to use my own hostname

[ OK ]
Unable to update the static FcBlanks: 0x0600
Unable to update the static FcBlanks: 0x0601
Unable to update the static FcBlanks: 0x0602
Unable to update the static FcBlanks: 0x0603
Unable to update the static FcBlanks: 0x06dd
Unable to update the static FcBlanks: 0x070f
Unable to update the static FcBlanks: 0x2028
Unable to update the static FcBlanks: 0x2029
Unable to update the static FcBlanks: 0xfff9
Unable to update the static FcBlanks: 0xfffa
Unable to update the static FcBlanks: 0xfffb
GdkPixbuf-WARNING **: Cannot open pixbuf loader module file '/usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders.cache': No such file or directory

This likely means that your installation is broken.
Try running the command
  gdk-pixbuf-query-loaders > /usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders.cache
to make things work again for the time being. at /usr/share/perl5/Debconf/FrontEnd/Gnome.pm line 148.
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
Use of uninitialized value $destinations in scalar chomp at /var/lib/dpkg/tmp.ci/config line 221.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/Client/ConfModule.pm line 121.
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
Use of uninitialized value $mynetworks in scalar chomp at /var/lib/dpkg/tmp.ci/config line 285.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/Client/ConfModule.pm line 121.
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
Use of uninitialized value $protos in scalar chomp at /var/lib/dpkg/tmp.ci/config line 387.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/Client/ConfModule.pm line 121.
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
postconf: warning: valid_hostname: invalid character 10(decimal): sebastian-VIO?sebastian-vio.rainworks.local
postconf: fatal: unable to use my own hostname
dpkg: error processing archive /var/cache/apt/archives/postfix_3.1.0-3_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1