We are having issues sending emails to multiple @mac.com accounts from various domains using different setups and via different ISP's. I.e. going through Exchange, POP.
It appears that the issue is intermittent and it looks like the email gets through but they don't arrive. We don't get a bounceback and it doesn't appear to be related to just new messages or replies. New messages seem to get through but replies or forwards with similar content don't. I have seen some examples where new messages don't get through either.
Initially we thought the issue was when sending emails hosted with fasthosts not getting through but it looks like others are affected as well.