Outlook Sender's Emailsl Blocked. Works with Gmail As Backup. ERROR MSG. FROM MICROSOFT OUTLOOK RE PROBLEM

Emails from an important sender are blocked by ATT email system rejecting emails from this sender who uses Outlook email system.  Problem started ~ 5/9 when only emails without attah's were delivered to me.  On 5/13, no emails from this sender got through. 

Following is a clip of the error message sent to sender from Microsoft Outlook:

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