Replies to an att.net email account come back as undeliverable due to the recipient's email system refusing to accept a connection from your email system.

Question

Can you please help me and my recipient troubleshoot an issue where I cannot reply to his emails?

I am replying to someone that has an att.net email address, from my outlook.com address, and I keep getting an undeliverable message saying:

Your message wasn't delivered. Despite repeated attempts to deliver your message, the recipient's email system refused to accept a connection from your email system. Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system is refusing connections from your email server. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem. For Email Admins No connection could be made because the target computer actively refused it. This usually results from trying to connect to a service that is inactive on the remote host - that is, one with no server application running. For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?Linkld=389361