For Outlook users:
For employees, the primary way to submit reports of Spam and Phishing messages is by using the PhishNotify button in Outlook.

Rarely, , in some cases you may need to use Outlook's older Report Message plug-in. This tool is available in Outlook via the ribbon, under the Protection section:

For Outlook on the web users:
For students, the primary way to submit Spam and Phishing email messages is by using the Report Message add-in for outlook on the web.
For more information, see Use the Report Message add-in.
Or:
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Create a new, blank email.
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Address the email to the Microsoft team that reviews messages as follows:
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Copy and paste the junk or phishing scam message into that email (as an attachment).
- Click Send
For Mobile users:
To report an email as phishing or junk:
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Select the email you want to report.
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Tap ⋮ at the top right of the screen.
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Select Report Junk from the dropdown menu (on some phones this option may be "Report Message" at the very bottom of the menu).
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Select Junk, Phishing, or Block Sender as needed.

Reporting Text Message (SMS) Phishing/Spam:
Text message phishing and spam can be reported via your text messaging app or via a government-run text recipient. Please see the FTC's guidance on reporting Text Phishing or Spam messages below:
https://consumer.ftc.gov/articles/how-recognize-and-report-spam-text-messages
References:
https://technet.microsoft.com/en-us/library/jj200769(v=exchg.150).aspx
https://docs.microsoft.com/en-us/office365/securitycompliance/submit-spam-non-spam-and-phishing-scam-messages-to-microsoft-for-analysis
https://docs.microsoft.com/en-us/office365/SecurityCompliance/report-junk-email-and-phishing-scams-in-outlook-on-the-web-eop