On Monday, April 26th, 2021, Apple had an outage with their mail servers servicing the @icloud.com, @mac.com, and @me.com domains. During this outage, messages sent to these domains hard bounced. We observed hard bounces from 4:00am PT to 11:30am PT on the 26th.
Our Engineering team has cleansed all faulty hard bounce data from this outage to prevent any further impact to the ability to send to these addresses.
If you have any questions about this, you can contact support here: https://connect.act-on.com/hc/en-us/requests/new