Aperture supports OS X Mail and Microsoft Outlook 2011 or later. Follow the steps below to specify which you'd like to use in Aperture's Export preferences.
Follow these steps to set the the mail client that Aperture uses when you choose Share > Email.
Choose Aperture > Preferences
Click the Export tab along the top of the Preferences panel.
Click the "Email using" drop-down menu and choose either Mail or Outlook.
If you use a different email client, you can still email photos from your Aperture library by dragging and dropping them from the Aperture Browser or Viewer into an email message. Some applications may also allow you to access your Aperture photos when adding attachments by clicking on Photos in the sidebar of any standard Mac OS X "Open" dialog box.
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