Thanks for the replies. I like the modern app, but unlike Outlook, there's no way to disable saving a copy of sent messages. My provider's SMTP server already saves sent messages, resulting in duplicates in the Sent folder.
if you have a POP account, you cannot use the mail app right away, but there is a workaround, using the www.outlook.com page.
Go to outlook.com and create an account, then go to the cogwheel in the upper right corner > Options > Import email account.
On the next page select Other email provider > (in the pop-up the "advanced options") "My provider does not support IMAP > choose one of the options:
a) import from mail client like Outlook, Outlook Express or Windows Live Mail (might also work with Thunderbird and others, but I didn't try)
b) import from pop3 server... (imports only the inbox)
Once that link is setup, you should receive your pop mail in Outlook.com on the web, which syncs easily with the mail app. Just open the app and enter your Outlook.com credentials. From that moment on, you can use your Mail app and Outlook.com on the web to manage your mails.