If you're building services like this, it might make sense to add some premium features that can only be paid for with NULL. That way, they would truly become part of the ecosystem.
Thanks for trying mail.nullnet.sh and for the feedback, glad you find it useful!
You're absolutely right, and that's exactly the direction we want to go: the services should be tied to the NULL coin and give it real utility inside the ecosystem. Extended mailbox lifetime paid in NULL is a great example of that.
The problem right now is practical: NULL isn't listed on any exchange yet. Until it is, payments in NULL can't be converted into anything, and running these services (servers, infrastructure, development) has real costs. So for now, any paid feature needs to be paid with something that has a market price, so we can actually fund the project.
Once NULL gets listed, we'll add NULL as a payment option for premium features, so every service on nullnet.sh becomes a real use case for the coin.
Thanks again, this kind of feedback really helps!