Refers me to https://support.apple.com/en-us/HT204137

When trying to email any other iCloud account, I receive a notification "Undelivered Mail Returned to Sender" user does not exist (in reply to RCPT TO command).

When in fact the user does exist when emailing from Gmail, Hotmail and all other accounts.

Reporting-MTA: dns; relay.xxxxxxxxxx.net

X-Postfix-Queue-ID: 6BXX13XXXDF

X-Postfix-Sender: rfcXXX

customer.service@xxxxxxxxxx.com and refers me to https://support.apple.com/en-us/HT204137

Q. What I don't understand is my own personal iCloud account receives these emails fine, and all other email accounts such as Gmail, Outlook etc do too, it's just iCloud accounts that seem to be rejecting any emails sent from our Customer Service email address.

Any ideas/solutions here please?