When using "apt", how do I avoid getting postfix error 75?

Question

I just upgraded my system from Ubuntu 20.04 to Ubuntu 21.04 and it came up with this error (as a graphical error window): "installed postfix package post-installation script subprocess returned error exit status 75". It then aborted the upgrade and reverted back to 20.04. The last line in the terminal of the updater read: "Errors encountered while processing: postfix"

Now, whenever I try and run apt, it gives me this error (for example this when I run sudo apt install git):

Postfix (main.cf) configuration was not changed.  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: fatal: bad string length 0 < 1: mydomain = 
dpkg: error processing package postfix (--configure):
 installed postfix package post-installation script subprocess returned error ex
it status 75
Processing triggers for libc-bin (2.33-0ubuntu5) ...
Errors were encountered while processing:
 postfix
E: Sub-process /usr/bin/dpkg returned an error code (1)

To solve this problem, I have already tried to modify myhostname in main.cf to only have one dot but this has seemingly done nothing.

If you need any more information please don't hesitate to ask. Thank you in advance for your help!

Answer 1

I know this is an old question, but I found a solution that seemed to work (I was getting the exact same errors you were . All I did was purge and reinstall it:

sudo apt purge postfix
sudo apt install postfix

I had not used it before getting this error, so I am fine with completely removing it.