Help with unblocking an email address

I have a friend who uses iCloud for his emails. I use em client on android, Windows and iPhone iOS my sent emails are received by him. However any emails that he sends to me are never received in any mailbox. I assume something is blocking his address. I am unable to find any filters in either Gmail or live mail. I have removed all data associated with his address in contact lists from all my devices and reentered it again ensuring is correct with no change in behavior. I am mystified.
If someone can help it will be much appreciated.

Hi John,

If I understand correctly: your friend uses Apple’s iCloud service and you use the eM Client (on different devices) to access your account, which is with Gmail.

Is your friend writing directly to you or only in response to your messages? It’s worth having them trying writing to you directly from scratch. If they write in response, I suppose it’s worth checking the reply-to address is correct.

With your own account, it is worth signing into webmail (i.e. access your account via a browser instead of a standalone client) and check the Junk folder and any server-side rules.

Thanks eMDash for your suggestions.
When my friend originates or forwards an email to me it fails to arrive in the Hotmail account.
However messages to my Gmail account from him are received just fine.
Something in Hotmail is blocking his messages.
If someone knows how to unblock a specific email in Hotmail it will be much appreciated.
Thanks, John.