I found my answer. Using "Inspect Elements" I found the column numbers and the display highlighted the column name.
If you are wanting to get rid of the email column try going to Site Administration > Users > Permissions > User Policies then scroll down to Show User Identity and remove the checkmark for email.
AL
Although I had already hidden the e-mail header using the line of code I had shown, plus one for the column, when I hid that option as you indicated, the e-mail column was no longer available in the grade sheet but then all columns from c3 and up shifted down by one. Columns/headers I had previously hidden above C3 shifted their hiddenness / visibleness to other columns. Hmmm Dilemma!
What other locations does your suggestion have effect?
I am not sure which method I should use.
Granted, bit I still would like to know how far reaching is the Moodle Setting.
Does it affect any student use of E-mail?
I've never tested this, but to me it's a visibility setting. Students can still message each other, for example, and the messages will go to their email boxes, but they will not see each other's emails onsite. Teachers most likely won't see the students' emails in the various reports etc.
Perhaps someone who has played with this option will add more.