What about adding an option to reply to emails (only reply, not send to arbitrary address)? I think none of temp mail services has it.
The moment you allow outbound traffic, you risk being weaponized as an open relay for spam. To implement a safe 'Reply-Only' feature, Mephisto would need a sophisticated validation layer that cryptographically links the outbound reply to a specific, recently received message ID. Even then, rate-limiting would have to be extremely aggressive. For now, staying receive-only is a deliberate choice to protect the service's reputation and ensure 100% uptime.