If they mean usernames and passwords for third party mail servers, then it is notable, since a traditional email client doesn't do that. They only send the cred

If they mean usernames and passwords for third party mail servers, then it is notable, since a traditional email client doesn't do that. They only send the credentials to the server they're authenticating against, not also to the email client vendor. That it's happening as plain text (notwithstanding TLS on the transfer) distinguishes this from some kind of credentials-sync system (think: LastPass or Keychain) which wouldn't necessarily need to have the credentials in plaintext to function.