iCloud Mail logo 2020Apple iCloud Mail blocks outbound emails (silent filtering) and inbound emails (spam filtering) containing specific, blacklisted strings. iCloud Mail screens the content of the email before sending it. If the content or the recipient’s email address contains a specific string of letters, iCloud mail prevents the mail to be sent and/or delivered. In my case the string “” was blocked, it did not matter if this string was used in the body of the email, in the subject line or in the email address.