While upgrading 20.04 to 21.04 I got this error
"Upgrade to 21.01 error
installed postfix package post-installation script subprocess returned error exit status 75
"Setting up postfix (3.5.6-1) ...
So after it completed the upgrade I ran the fixes built into the welcome>software>"fix" section of the welcome screen except The "packages". That returned this error
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: warning: valid_hostname: misplaced delimiter: HP8000..
newaliases: fatal: file /etc/postfix/main.cf: parameter myhostname: bad parameter value: HP8000..
dpkg: error processing package postfix (--configure):
installed postfix package post-installation script subprocess returned error exit status 75
Processing triggers for libc-bin (2.33-0ubuntu5) ..."
I am not sure which editor I should use to edit the configuration files. I tried to install nano, but that failed. Maybe I am reading this wrong, but is the error complaining about the computer name? Or a limit upon that name HP8000=1 but should be 0?
Hello jmarkus
My understanding is that with Ubuntu and its flavours you can not upgrade from 20.04 to 21.04 directly. You must go 20.04 -- 20.10 -- 21.04. It may be that from where you are now a fresh install is the best way forward. 
Thanks alinejohn. I had run the update, and then I ran it again, because lately on the second go it offers to remove old kernel headers, and unused configuration files which frees up space. It did that - then offered to upgrade to 21.04. The thing is - everything is still working, but I know this error has to be cleared and is likely exactly what the error msg is saying. Probably just a few characters are wrong. I have edited config files in Arch - what do people in Ubuntu use to edit their config files?
Hello jmarkus
(1) nano is installed by default, you should not need to reinstall it. If it is not installed, this may be another pointer for considering a fresh install.
(2) just to be clear, are you looking for a terminal text-editor, or a GUI text-editor? 
You can use any editor you're familiar with. Of course, if you can't install packages right now due to broken dependencies, then you're limited to the text editors that are currently installed.
I know that Ubuntu MATE comes with at least three functional text editors: nano (which you claim isn't installed -- I find that very hard to believe since it's usually marked as "Essential", but let's say you're correct), vi (actually a minimal version of the classic vim text editor), and Pluma (the default MATE text editor).
I would say use whichever you're most comfortable with. Trying to group Ubuntu MATE users' text editor preferences together is about as absurd as saying that all millionaires like to drive black cars -- it's little more than a stereotype. If you've used Arch in the past, I hope you understand that Linux is all about choice -- and text editors are at the heart of it, in many ways: The easiest way to start a heated debate among die-hard Linux / UNIX users is to post a forum or newsgroup thread that simply states "emacs is better than vim".
I did not know nano was installed by default, but I tried to edit through nano and got an error message - then tried installing it and got another error msg. Now the terminal has changed the default user to the correct name followed by a tilde (~) instead of the $, and I can not enter any characters. I will mess with it for a few more days (it is dual booting win10), but it looks like a clean install is likely. Thanks for responding - I will say that even "broken" all the applications I use are still working just fine.
I found that the "edit /etc/postfix/main.cf" was impossible, because it was not on the machine. So I installed 21.04 over the old install, and setup everything all over again. Interestingly - it kept working right to the end. Thanks for the suggestions. It is running great right now.