You're being naive thinking you can run an open spam service with no consequences. Maybe you don't care about abuse but the hosting does and will act on the recurring complaints.

Seriously, cut the ability to reply to emails and that should be fine.

There's no use case to send replies for an anti-spam. Never seen a registration process that required to reply to complete the registration.

Tip: The google postmaster tool can show you the reputation of your domain and how much of your outbound emails are going to spam. That shall give you an idea how well it's abused. https://www.gmail.com/postmaster/