Recent Outlook User Experience updates include more generous spacing and padding for a cleaner, more modern look, and to make the message list easier to scan.
If you prefer tighter spacing in your message list, task list, and around the To, Cc, Bcc, and Subject lines when composing a new email message, do the following:
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If you're using the Simplified Ribbon, select View > Use Tighter Spacing.
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If you're using the Classic ribbon, select View, and in the Layout group, select Use Tighter Spacing.
