ATT (currently.com, sbcglobal.net) is blocking emails sent from icloud.com account

Mail Delivery System says this--- This is a system-generated message to inform you that your email could not

be delivered to one or more recipients. Details of the email and the error are as follows:

Various IP addresses and hosts are identified, and it says that that the iCloud.com account is blocked. But no action was taken on either of two ATT accounts to block any email address or IP number.