I removed your checkbox objects. The Checkbox objects make this more complicated than it needs to be.
What I did was create a Worksheet_SelectionChange event that inputs an "R" if it's empty or input empties the cell if "R" is already there.
I then changed the font of the cells to Webdings 2 so that the R shows up as check box. Bow when you click the cell it will either check it or uncheck it. This allows me to check if an email needs to be added by checkinging the column for a value of "R". Hope this makes sense.....
See attached......