This error indicates that the email account has been blocked because the account has been sending emails to a large number of invalid addresses. Spammers often send to a large number of invalid recipients. A high number of invalid recipients will harm the IP reputation.
To unblock the account, please clean up your mailing list by verifying the email addresses you are sending to. You could use a website such as http://www.verifyemailaddress.org/ or http://mailtester.com/testmail.php for address verification.
Best practices to avoid this problem include the following steps:
- Reduce the number of invalid recipients on your list by using double opt in (includes confirmation).
- Remove invalid recipients from your list immediately. You will always have some invalid addresses due to people changing email addresses, but the lower the number, the better your reputation.
- Only send to subscribers who have opted in to your list. Your contacts must subscribe to your emails and have valid email addresses. If they don’t and then mark your email as spam and we receive a feedback complaint, your account will get suspended temporarily or permanently.
Once the mailing list is cleaned up and the number of invalid addresses goes down, you will be able to send mails automatically without any intervention from MailChannels.