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Windows has a notion of what your “default” email client is.
An installed email program can be set as the default to access your web-based email service.
Web-based email can typically not easily be set as the default for sending, with the exception of Gmail if you use Chrome. (The process is slightly different for Windows 7.) You can easily tell Windows 10 what installed email program to use as the default for sending mail. Windows has a notion of a “default” email program used to send mail, most commonly in response to a webpage’s “mailto:” link.