We've send you the invitation for a custom plan which allow email validations upto 10000 per day.

Not working with Soundcloud

Larger plan

API not working with Soundcloud

You can easily unsubscribe on the API page. You don't need to delete your mashape account, as you could also be subscribed to other API through the account.

There should be some gap between requests. if you're verifying too many emails too frequently for a single mail server, then obviously it's going to be blacklisted.