@magnusvhendin I do not think this would be wise. Most (all?) SMTP relays will require a sender address before accepting/relaying mail. Hence, I'd suggest you create an address such as "email@example.com for such purposes. Then associate that email address with a user. Maybe one you don't use for other stuff?
Or maybe I do not understand you clearly: You ARE already using valid value for email address and just want to omit UID? Which seems like easy enough for the dev inclined. But I wonder what this really achieves?
Anyways, just a couple thoughts since none have yet been forthcoming... Good luck.