iCloud Mail is a service that Apple provides. Most often, such addresses are created automatically by users and are rarely used for active work with mail. We do not recommend sending mass mailings to such addresses. It is better to offer the user to create a mailbox on a popular service — Gmail, Yandex or Mail.ru.
If the messages are tagged as spam on iCloud, then in the message sent to the user with the mail on @icloud.com, @me.com or @mac.com, you will see the «Blocked» error in details:
You need to follow the link provided in the error details and follow the instructions.
Please note that in iCloud most of the checks are automated.
If you have problems with delivering letters to iCloud, we recommend that you stop sending messages to these addresses for a while.