An ISP or mailbox provider may prevent your email message from getting accepted due to authentication, reputation, spam failures, or any other cause that negates their spam and email policies. In that case, they block the email message.
The email of the recipient may block inbound mail from our server. This may be a result of blacklisting. A mail block happens when the receiving server blocks an email totally, i.e., 100% rejection without trying to deliver the email into the inbox.
A block may be due to a blacklisted reply-to-address, a temporarily-blocked sending IP and/or domain, or the configuration of the receiving server to only accept whitelisted senders. Whichever way, the only situation where you may resend a blocked email is if the bounce is a result of our sending domain and/or IP being blacklisted or blocked.