"Send to mail recipient" is dimmed out on my tool bar

I often use the "Send to mail recipient" option in my Excel 2007 to send a
single sheet in an email, rather than attach the file. But this option is now
dimmed and won't run. I've deleted it from the toolbar and reinstalled it,
rebooted, etc., etc. All the other options in the tool bar continue to
function as they should. And the option to email an excel file still works as
it should. I must have done something to disable this option, but I can't
figure it out. Thanks for any help or suggestions.

Mike