that's the "0/zero" key on the number pad...

to test, turn Num Lock off
write a new email
click one key 10 times
bring the cursor to the middle of the line you just typed
click 5 times on a different key
if you have 10 letters, click Ins, then click Num Lock (this will turn the Ins key to a Zero Key, and also lock it)

hope this helps