This probably wont help and the suggestions by Tim is good, but sometimes you may want to actually modify the HTTP headers passed to the WCF service. You could then inspect the headers and obtain custom information such as email addresses.
An interesting article is
Add Custom Message Header in WCF 4 Calls[
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Another way (but probably a poor but dirty way) would be to replace all "." (dots) in the email with the character "~" (squiggle) and then in the service replace the "~" (squiggle) with "."'s. Again these approaches are probably not "best practice" but there are ways to skin a cat!!!