Preparing to unpack .../python3-urllib3_1.26.5-1~exp1ubuntu0.1_all.deb ...
Unpacking python3-urllib3 (1.26.5-1~exp1ubuntu0.1) over (1.26.5-1~exp1) ...
Setting up python3-urllib3 (1.26.5-1~exp1ubuntu0.1) ...
Setting up postfix (3.6.4-1ubuntu1.1) ...

Postfix (main.cf) configuration was untouched.  If you need to make changes, 
edit /etc/postfix/main.cf (and others) as needed.  To view Postfix 
configuration values, see postconf(1).

After modifying main.cf, be sure to run 'systemctl reload postfix'.

Running newaliases
newaliases: warning: valid_hostname: numeric hostname: 142.212.20910000
newaliases: fatal: file /etc/postfix/main.cf: parameter mydomain: bad parameter value: 142.212.20910000
dpkg: error processing package postfix (--configure):
 installed postfix package post-installation script subprocess returned error exit status 75