Hi!
It's common but I don't do it. Actually, it depends on what you want to achieve. I always want to give each of my students a chance to write an e-mai. Some of them find it harder than others, and they can hide behind the braver ones. But, on the other hand, if you want them just to make a contact with a class, that's a good idea. However frightening to your students one-to-one exchange may look, they all feel great after their first mail, believe me.