It was working well even after doing an in place upgrade to the new build\Creator's Update, but since today the images won't load, just red X. The option about downloading external images is enabled.
Resetting the the mail app worked. Forgot to update the post, sorry.
Go to Windows Settings, then Apps. There find Mail and Calendar and Advanced Options-->Reset
It will reset the app and erase the email accounts, but you will not have to reinstall Windows and adding the accounts to the mail app client is easy now.
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