Right! I've spent some time on the phone to Apple today over this issue and have been dealing with a lovely Senior Tech lady. It does seem that noreply@geocaching.com (or the associated IP address(es)) has almost certainly been added to an Apple blocklist, therefore emails from that address to any Apple email accounts (@icloud.com, @me.com etc) will bounce back.
There is nothing we can do on our own accounts or devices to fix this, Apple need to lift the block.
This has been elevated for the techy people at Apple to look at although they're dealing with a backlog from Christmas so it may take a couple of days. The lady that I've been dealing with has promised to ring me back with an update when she has one.
In the meantime she suggests that people should contact Apple if they are affected... the more people report it, the better, basically. The thread over on the Apple forums which has been linked to here has been helpful in flagging up the issue as the Apple people can read it (although they can't reply).
So, if you are affected, please post that both here and on the thread in the Apple Forums. In addition, please go ahead and contact Groundspeak directly via the contact link on the bottom of the homepage so that they realise how widespread the issue is. Although it's not Groundspeak's "fault", it would be nice if they tried to fix it!
Most importantly, please contact Apple. It's free from landlines in the UK... 0800 1076285
You'll need to explain that you are having this known issue, they have an internal thread about it on their forums, or you could point them at the apple forum thread HERE. It will obviously take time for all the staff on the phones to become aware of the problem, so please be patient but insistent
I must say thanks to the contributors to this thread and over on the Apple forum, it's helped to tie everything together to figure out what's going on, lets hope we can sort it!