There is such a thing as an offer that???s too compelling, though, warns Elaine O???Gorman, vice president of strategy for Atlanta-based e-mail services provider Silverpop. ???If you???re going to incent at all, you want to make sure it???s rich enough that people go to the trouble of digging out their friends??? e-mail addresses but not so rich that they???re going to dig out the e-mail addresses