Tips

If you’ve recently received the following email bounce message when attempting to send a message to an email address hosted on Microsoft 365, this guide is for you: 550 5.7.1 Service unavailable, Client host [x.x.x.x] blocked using Customer Block list AS(110) This error indicates that the sender's IP address is blocked by the recipient's Microsoft 365 account, preventing the email from being delivered. Here's how you can resolve this issue: Step 1: Access Microsoft Defender If you have access to the recipient's Microsoft 365 account, start by logging into Microsoft Defender. To do this, navigate to Microsoft Defender and log…