I’d noticed my Preview.app (Yosemite) share menu had Air Drop and Message, but not email.
I tried Apple’s recommended fix for missing share options: If Share options or Markup is missing after upgrading to OS X Yosemite or El Capitan but it didn’t help. I got my clue looking at System Preference:Extensions:Share Menu; it showed a Chrome icon for Mail.
In Mail.app I changed default mail handler back to Mail.app. That returned the Preview Share Extension for Mail.
PS. It’s a good idea to review Extensions periodically. I did some cleanup there.