Trouble receiving from iCloud

[#1]

I have 4 mailboxes set up on our PN email for the family. One of them seems not to be able to receive mail from iCloud email addresses (in fact I've seen the same mail sent to several of our mailboxes arrive successfully in all except the one that's having trouble).
I reported it a long while back to PN who suggested setting up a white list, although with the problem being limited to one mailbox it didn't seem to me that a whitelist at account level was going to make any difference - and it didn't. It's become an issue again because there's a need to receive emails from a few people with iCloud addresses.
Has anyone come across anything similar?

[#2]

Can't say I have. Do the senders receive a non-delivery notification? I assume other emails sent to the same mailbox arrive without issue?

[#3]

No, there's no non-delivery notification, they just never arrive. We're using Outlook, it's exactly as though there's an inbox rule that deletes iCloud messages as they arrive (there isn't one!). There isn't a problem with any other senders as far as we know, just ones from iCloud. Baffled...

[#4]

And it only affects a single mailbox? Very odd
Would you mind sharing the first two characters of the mailbox it affects?