Hi @Anthony
Based on your description, it looks like you've enabled autocomplete. This feature is designed to suggest email addresses when you start typing based on your previous usage.
Check if Auto-Complete is Enabled: Go to File > Options > Mail and ensure that the ‘Use Auto-Complete list to suggest names when typing in the To, Cc, and Bcc lines’ option is checked.
For more details, you can refer to: The Outlook AutoComplete list.
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