Mail app losing emails

If, as you imply, if the Mail app is corrupted, the only way to reinstall it is to reinstall the operating system. Mail is installed as part of the OS and cannot be separately erased or installed. As an aside, I long ago stopped using Mail and Contacts. I use the Thunderbird mail app sandlike it very much. It is free and has excellent support. You might want to give it a try.

Thunderbird is a good App for a Desktop only! But as far as I know it does not have any mobile support for IOS or Android. So on your phone or a pad you will need to find something else. But on the Mac App Store there is Spark (Macos, IOS) and Edison (Macos, IOS, and Android not sure about Windows) and they will Sync with all your Online Devices. Both are free but there is a in App Purchase if you so wish. I myself use the Free Version of Edison and it does everything I need.

You make an excellent point here and that it is important when evaluating an email app to understand what features and needs you have. (Mine are quite simple.) When I was looking for an email app to use instead of Mail, I made a spread sheet which listed various features and costs, then I would visit the various web sites and check which ones an app did or did tot have. Only by doing this was I able to see what ultimately suited my particular needs. I found that there are a lot of email apps out there which can and do support many individual needs. They are definitely not all alike.

You're welcome and thank you for your input.