As said in the tagline, your service is grouped with others that serve _disposable_ emails.
Safe use cases for services providing disposable emails are somewhat slim. If you sign up using a disposable email, and later the account ends up being less disposable than you thought, for example because it contains some personal data from you, then you're taking a huge risk in not either using an email service that's too-big-to-fail, or your own domain.
It's easy to forget this, and use disposable emails for everything, so the users' biggest risk is you exiting or the service going out of business with the domain hijacked.
If you could somehow reduce the risk of using 33Mail to lower than it is to mess with big email services or custom domains - yes, I saw you can use custom domains with 33mail, but you still have to keep it renewed yourself I assume - then I'd be happy to pay whatever you ask for the service.
For how to guarantee to users that you will outlast the services they sign up for, I have no answers.
As we've learned many, many times, even recently with things like Oculus, promises mean nothing.
Other than that, I'd love to see a feature for having the option to generate anonymous aliases. That is, aliases in the format of @ for cases where you don't want to give your username to everyone.
Actually, having those two, a guarantee that your service is safe to use and unlimited anonymous aliases, would make 33Mail unique and something I'd actually use instead of custom domains. Because with custom domains the domain still links all my accounts together, becoming both a privacy and a security issue.