Not worry about it? It's not like he can see it anyways, it's encrypted on the client.
If someone sends an encrypted email containing illegal content, is email then considered harmful?
You design a system that gives you (the hoster) zero ability to inspect, modify, block, censor, restrict any of your users' content and in doing so you're allowed to abdicate all responsibility for how the system is used because you're literally incapable of controlling it. Pure infrastructure with none of the messy real-world bits.