5 Oct 2021
Sometimes trying to boot into Ubuntu 20 from VirtualBox just results in a black screen with blinking cursor. The last known version of VirtualBox to work was a "test" version, VirtualBox-6.1-6.1.27_146688_test_el7-1.x86_64.rpm but now it fails.
I then installed VirtualBox-6.1-6.1.27_147252_test_el7-1.x86_64.rpm and the VM started up.
For all notes below, the new Ubuntu 20 VM has been installed and tested. Also migrated Unifi controller to it.
Upgrade 18.04 to 20.04
After following these steps and booting into new kernel, machine would not start
Booted into old kernel, and ran an update for new packages. New kernel was downloaded and installed.
Had a new kernel not been available, I suspect reinstalling the original kernel that was failing would have fixed the problem
Fix /swapfile
Change hostname
Gmail SMTP on Ubuntu 20
Install postfix and choose "Internet site"
Accept suggested FQDN
Don't add another user for /etc/aliases (root is already there)
For comma-separated list of domains, accept defaults
Accept defaults for everything else, such as synchronous mail, network blocks, etc.
Follow rest of instructions
Send test message
Disable firewalld on Ubuntu: https://www.cyberciti.biz/faq/linux-disable-firewall-command/