Below are the possible reasons that you may not be able to send emails:
Reason 1: You have not verified your domain.
You need to verify the ownership of the domain before you can send emails.
Reason 2: You have purchased the email ids
Our system constantly checks your subscribers list against various purchased database of email ids. In case, your subscriber list matches with the purchase list of email ids, you will not be able to send out email. Your account may also be suspended. Hence, we advise users and organisations NOT to use purchased database of email ids to create a mailing list.