On the Mail issue...
Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…
Then you may need Ways to securely access AOL Mail…
https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in
Or it may just need an App Password...
https://support.google.com/mail/answer/185833?hl=en-GB
Yahoo has gone to this too...
https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html
Yahoo App password…
Generate and manage third-party app passwords
Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app.
https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html
Using app passwords with apps that don't support two-step verification (microsoft.com)
On the OS...
macOS High Sierra - Technical Specifications
General Requirements
- OS X 10.8 or later
- 2GB of memory
- 14.3GB of available storage to perform upgrade*
- Some features require an Apple ID; terms apply.
- Some features require a compatible Internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:
- MacBook (Late 2009 or newer)
- MacBook Pro (Mid 2010 or newer)
- MacBook Air (Late 2010 or newer)
- Mac mini (Mid 2010 or newer)
- iMac (Late 2009 or newer)
- Mac Pro (Mid 2010 or newer)
macOS High Sierra - Technical Specifications
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
macappstores://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12