In this blog post we will talk about the most common error “550 permanent failure for one or more recipients”. This SMTP (Simple Mail Transfer Protocol) error occurs when the recipient mail server blocks email. There are various reasons for this such as restrictions, blacklisted IP addresses, bad DNS records, etc. We cannot know what the exact issue is. Therefore, we need to investigate to confirm if it is related to the sender’s or the recipient’s mail server. In the next paragraph, we will explain what steps need to be taken to detect what makes this issue appear.