I believe that your Launch Services database is corrupted. Apple has this solution. Shutdown completely afterward, and then Power up.
Once you have done the preceding, check first in the System Preferences : Extensions panel : Share Menu to see if Mail is there now. If it is, select it, and it should now be available to you in Pages v5.6.1.
If the Apple solution above did not restore the Mail entry in the System Preferences, then use mine, which has helped others this fall. Triple-click the scrollable text and paste into the Terminal app. Press return. Upon completion, shutdown, and then Power up.
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -seed -domain local -domain system -domain user