Sharing photos
If I make Outlook my default email program it doesn't come up in the Share menu in Photos.
iMac 21.5″, OS X 10.10
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You can only share with mail applications, that offer a sharing extension in "System Preferences > Extensions > Sharing". I have no longer the Microsoft Office applications installed, so I cannot check, if the most recent version of Outlook includes a Sharing extension.
You may want to check the System Preferences and enable the sharing extensions, that are available.
But even, if a sharing extension is available, they option may vanish from the Share menu, if you have too many photos selected at once.