In order to be able to send mail via the 'Sales Order' tab you need to enable 'Print View' in the 'Profiles' settings.

It took me half a hour to figure this out as it's not clearly labeled. (why would the send mail permissions be labeled that??!)

Also, enabling 'Print View' also allows the users to print out all the leads, which is a major security breach for any company.

I'm hoping this gets fixed asap.