So far I haven’t seen a comment point this out or suggest similar, so let’s say that instead of trying to maintain an application level list of email addresses that is used in a breach (or for other reasons), rely on the exercising service (email) which by formerly sending a verification email, has a record of the destination at least in a log, and maybe during registration placed in a “verified member” list, all more or less managed within the mail service.

Article 34.3.a seems to disavow the data processor of such requirement.

  The communication to the data subject referred to in paragraph 1 shall not be required if any of the following conditions are met:
  (a) the controller has implemented appropriate technical and organisational protection measures, and those measures were applied to the personal data affected by the personal data breach, in particular those that render the personal data unintelligible to any person who is not authorised to access it, such as encryption;

https://gdpr-info.eu/art-34-gdpr/