I see it’s normal for me. If an account is inactive, the user perhaps has created another account or not using Google services anymore its money saving if we consider all personal data, ie 15 Gb per user, which are stored and maintained without reason.
I’m using Gmail but I have also some other accounts from internet providers and a electronic certified mail which can be used for official communication with institutions or companies in Italy. I have also a proton like service with another address… Everything controlled with FairMail which I recommend as excellent mail client. Basically there’s no need to use one or the other but you may also use both depending of the type of message, I guess
You may compare both as, is possible to share the same profile between the 2 apps. Actually I rely on Thunderbird, as I use it since the beginning, and it has the functions I need. Perhaps I will give a try on the Betterbird as well.