How to Stop Outlook From Typing Over My Text

You can force Outlook to recognize the Insert key for toggling Overtype Mode.
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If Outlook 2013 is overwriting non-selected text as you type then Overtype Mode must be enabled. This mode is a manual selection buried within Outlook’s preferences. Curiously, this feature can only be changed when accessing the Options menu through a message window. You can still see the option when accessing the Options menu through the main Outlook window, but Overtype Mode is grayed out and inaccessible. You also have the option to enable the Insert key to toggle Overtype Mode; otherwise, the Insert key is nonfunctional in Outlook.

Press "New Email" from the main Outlook 2013 window. Alternatively, double-click any email.

Click "File" from the message window and select "Options."

Click the "Mail" tab, select "Editor Options" from the Compose Messages section and click the "Advanced" tab.

Uncheck "Use Overtype Mode." Alternatively, check "Use the Insert Key to Control Overtype Mode" to enabled toggling Overtype Mode using the Insert key.

Press "OK" to save your changes and close the Editor Options window. Click "OK" again to close the Outlook Options window.