Thanks for your report.
The relevant postfix error is
---
Running newaliases
newaliases: warning: valid_hostname: numeric hostname: 192.168.10.101
newaliases: fatal: file /etc/postfix/main.cf: parameter mydomain: bad
parameter value: 192.168.10.101
could you please check your configuration in
Thanks for taking the time to report this bug and helping to make Ubuntu
better. We appreciate the difficulties you are facing, but this appears
to be a regular (non-security) bug. I have unmarked it as a security
issue since this bug does not show evidence of allowing attackers to
cross
