Managing personal Outlook.com account settings, security, and privacy
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If you are getting emails from what appears to be your email address or username, it does not necessarily indicate that your account is compromised or that somebody has access to it. It's called spoofing, they forge the "from" to look like your email or username.
First, check whether your account has actually been compromised.
Go to your Sent Items folder. If you don't see those messages there, they were likely spoofed rather than sent from your account.