Comment 2 for bug 1696809

From the log:
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

It fails on restarting the server on the upgrade, but it seems to be related to the configuration.
Now you seem to have:
PostconfMydomain: localdomain
PostconfMyhostname: sebastian-VIO.localdomain

I think the "-" is what it can't work with.
Probably because it has to fulfil so many standards with it (smtp and more) it limits that.