If you want a pretty easy and minimalistic mail app I suggest Mailspring, I tried it some time ago and despite a problem with synchronization it was nice.
I prefer Evolution. As well as using it for work email working from home until I retired for accessing outlook365, and also my @e.email that uses IMAP. It picks up settings straight away in respect of IMAP. Outlook365 being a work email address needed manual setup for incoming and outgoing but very easy to setup.
When you say that you didn't find the files are you saying about your files like photos, videos and games? A mail service shows you mail attachments on other devices once you log in to them with the same account, it's not a file sharing service like Google Drive. If you meant to see your files (not referring to mails attachments) you need another kind of service.
I used Thunderbird for several years. Until last year when updates started messing things up. I found Betterbird which is an independent fork of Thunderbird. I have been using it for about 6-8 months I think. It is much faster than Thunderbird and so far has worked great. It easily finds your settings from your email provider and sets them up. You just need to add your password and you are all set. it also allows you to import your Thunderbird profile directly into Betterbird. So your address book and other settings are moved easily.
Evolution (or GNOME Evolution, formerly Ximian Evolution and Novell Evolution) is an application that was simply put, integrated into the GNOME interface, and functions much like Microsoft Outlook.
If you like, you can use other mail apps like Mozilla Thunderbird, which I prefer.
Betterbird is not connected with Mozilla. As I posted, it is an independent fork from Thunderbird. Betterbird will ask if you want to import your old Thunderbird profile into Betterbird. If you answer yes it will look for your Thunderbird profile and import it. It does not import your old emails, only your profile. If you want to move your old emails you will have to do that manually.
I have try evolution, the app is good.
But i will used thunderbird, because i used it long time.
i know , when change mail app many problems with incompatible export/import.
But i respect other meaning too.