Step-by-step guide:
Sign in to Outlook on the web using this link: microsoft365.com and your 8-character username @uwaterloo.ca (e.g. myuserna@uwaterloo.ca) and password
Click the New mail icon
Start typing the name or email address of the recipient that you would like to remove in the To field until the recipient appears in the drop-down list.
Use the Down Arrow and Up Arrow keys to select the recipient, and then press the Delete key.
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