How to e-mail pictures from Photo using El Capitan

Select the Picture(s) you want to mail. Click the Share icon, select Mail.

Is it greyed out but checked?

In teh Mail application set Mail as your default mail reader

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To set Mail as the default email reader, open the Mail preferences and check the option

"Mail > Preferences > General: Default email reader: Mail.app".

Then restart Photos and Mail.

If that does not suffice, try this fix: If Share options or Markup is missing after upgrading to OS X Yosemite or El Capitan - Apple Support

https://support.apple.com/en-us/HT203129