For all premium memberships, important emails goes through premium email gateways. These emails will not deliver if your email gets blacklisted. The reason for email blacklisting is hard bounce within the past 14 days.
When a recipient address generates a hard bounce, our premium gateway considers that address to be temporarily unreachable, and blocks sending to it for a period of 14 days. Address blacklisting is entire premium gateway-wide. If a recipient address has generated a hard bounce for any premium gateway sender, then you will get this error message if you send to that address, even if you have never sent to it before.
For example, suppose Sender A emails email@example.com, but there is no user andrew at the domain example.com.This results in a hard bounce, and the address firstname.lastname@example.org is added to the SES address blacklist. For the next 14 days, any email to email@example.com from Sender B, Sender C, etc., will be blocked, and all these senders will get "Address blacklisted" error messages.
If you still face email delivery issue, please let us know the email id (that you experience issues with) along with your Appointy username at firstname.lastname@example.org and the tech team will check it for you.