Okay, all you need to do is:
Make sure the combo box has a linked Cell, you can set this by right clicking on the combo box when in edit mode and choosing format control. it doesn't really matter what cell you choose. lets say A1.
once this is done you can put a formula in the cell which you would like to display the email address:
formula: =Vlookup(A1,Sheet2!A1:B10,2,False)
A1 is the linked cell
Sheet2!A1:B10 is the range where the list is stored
2 is the column data to return
False is the match type.
If you need anymore help let me know.