How can I adjust iCloud security settings for email?

Hi,

As the title suggest, for some reason I can't receive emails from a client of mine. We've been emailing for weeks with no problem. However, the two people in my email thread said they now can't mail me.

They receive the following message:

Security or policy settings at iCloud.com have rejected your message.

As far as I am aware I've not changed any settings. It was fine for weeks until two days ago.

I've checked rules, nothing in there.

I've checked they are added to my contacts, they are, still can't send to me.

I've marked them as VIP, still can't send to me.

For a moment I got a message saying Update Your Forward To Email Address. Then it disappeared. I don't know I have a forward address on this account. This is standard Mac Mail/iMail (it's synched to my phone) that I've used for years.

I can email them and they receive it. They can not respond. (They could two days ago)

I can receive emails from other domains. It seems to be their domain (they are both on the same domain and can email other people but not me) which is all of a sudden blocked.

This seems to be a problem my side.

Can anyone please advise.

I really don't want to start messing with mail settings. As far as I'm aware I've not done anything my side.

their domain is not marked as spam/blocked.

Currently on a Mac Mini M2 Pro with Sonama 14.1.2 and iPhone/iOS 18.3.1

I really don't want to update anything as I'm set up with everything for work. Everything is running OK, don't want to meddle unless I really have to.

Thanks

[Re-Titled by Moderator]