Our company culture is very informal. We all use first names, and everyone is per du with everyone else. But I'm not really comfortable addressing my boss as "Lieber Thomas" in an email or letter - that seems a little too informal. "Sehr geehrter Thomas" just sounds ridiculous, though. I've been starting my emails with "Hallo Thomas" so far, but that doesn't seem ideal, either.
Is there a better way to (semi-)formally address people you're on a first name basis with?