Like a few others I performed an in-place upgrade of Ubuntu from 18.04 to 20.04 and then 22.04, and then ran the mailinabox installer to get version 60. I know this isn’t supported, but It works, and I’m able to send and receive emails just as I could before.
I’m guessing the reason this method isn’t recommended is the warnings you get in /var/log/mail.log after doing this. Both postfix and dovecot retained the old configuration files, which causes warnings about settings no longer being required in dovecot and backwards compatibility mode in postfix.
I’m pretty sure these are going to cause an issue in the future, so I’ve fixed them myself by performing a clean install on a new instance and then copying /etc/postfix/main.cf, /etc/postfix/master.cf, and /etc/dovecot/conf.d/10-ssl.conf back (this seems to have covered all warnings so far)
Other than that I haven’t had any issues