Trying to migrate Mail from Vista machine to one with Win10

For a relative who had for years been running Win Mail on her VIsta machine ...

She acquired a new machine with Windows 10 installed and I assisted with migrating all data over. Regarding the Mail system, I opened Mail on the Vista machine and clicked Export for all her Mail data, including Contacts.

Once the Win10 machine was booted, I opened the Mail utility and presumed I would be taken to a launch window that presented, among perhaps other things, an Import button. Not so !!!

To my surprise and disappointment, the first screen that appears is one asking me to select an account (within a drop-down window)!!

Is there no way to simply bring in the Exported Mail files to the Mail app in Windows 10 ??

Did you have a choice as to what format to export from Vista?

Are you locked into using the Windows 10 mail program rather than a desktop application like Thunderbird--for instance?

In my experience, exporting and importing mail between operating systems and various email programs has long been frustrating. You may have to rely on some type of "converter", which introduces another can of worms.

Here's some links on the topic:

Exporting emails from windows mail what format?

Importing mail and contacts from Windows mail in Vista

How to reinstate WinMail onto Windows 8 or Windows 10 | Windows 8 Help Forums

Backup Windows Mail Messages and Contacts in Vista