There is general storage in SY01200,but there are several places in SY01200 you might need to look (sigh), article here:
and they are sometimes stored in RM00106, article here.
There may be more in other modules, I'll add to this as I find them. Bottom line, you'll have to ask the customer to tell you where in GP they store email addresses and the above articles will give you the code to retrieve them.