MacOS – This is continually showing in the syslog. I know it’s mail generated but what

Are there any background applications or kernel extensions that could disable the popovers? For example KeyRemap4MacBook disables them.

What input method do you use? The popovers don't work with all input methods, or custom keyboard layouts that are not saved in the bundle format.

You could also try to run defaults write -g ApplePressAndHoldEnabled -bool true and reopen applications. Or delete ~/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist and restart.

Does it work on other accounts, or did it work in earlier versions of OS X? Is there a process named PressAndHold?

Related questions:

• Character Picker stopped working in OS X Lion

HSTS.plist is a Safari settings file related to HTTP Strict Transport Security. It is a mandatory setting for Mavericks and newer OS releases. For more information about this security policy mechanism, you can read up about it at Wikipedia:

http://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security