I can't imagine a situation where MailChimp is legally prevented from enforcing its own terms of use just because a third party pointed out the breach. Think of how exploitable that loophole would be.
I can't imagine that either, but this isn't a case between Stefan and Mailchimp, this is a case between Stefan and Nandini in Tort Court where the requirements are much lower:
1. The existence of a contractual relationship or beneficial business relationship between two parties.
Stefan is on mailchimp
2. Knowledge of that relationship by a third party.
You are inappropriately equating the termination of a contract with a breach (= a violation) of the contract. The Mailchimp terms of service allow the company to terminate the contractual relationship at any time [1] -- they did not breach the contract by choosing to terminate it.
[1]: https://mailchimp.com/legal/terms/ -- "3. Closing Your Account: You or Mailchimp may terminate the Agreement at any time and for any reason by terminating your Mailchimp account or giving notice to the other party."
> I can't imagine that either, but this isn't a case between Stefan and Mailchimp, this is a case between Stefan and Nandini in Tort Court
Well, it's not a case at all, and it's not likely to be one.
> Intent of the third party to induce a party to the relationship to breach the relationship.
Terminating a contract as provided for in the contract is not a breach of the contract. The only arguable breach is on Molyneux’s part by violating the ToS, and clearly Nandini didn't induce that breach.