Does this only affect Mailgun's customers? If these customers hold data of third-party – let's call them "end-users" – in Mailgun accounts, Mailgun could/should

Does this only affect Mailgun's customers? If these customers hold data of third-party – let's call them "end-users" – in Mailgun accounts, Mailgun could/should communicate the total number of individuals affected. "1% of our customers/users" can affect millions of individuals.