If TalkTalk were rejecting all incoming icloud mail messages then there would normally be a bounceback message. For example, a TT992 bounceback error code would indicate a rejection because of incoming spam from icloud mail servers. This was a recent issue and all mail was rejected including that from genuine originators that weren't sending out spam but just happened to be caught up in the flood of spam sent to TalkTalk Mail users.
First question is for your contacts and to know if they get a bounceback error and what the message says along with the TT error code.
Some icloud mail servers may be blacklisted for sending spam. In that case there may not be a bounceback error.
In either case it's up to Apple to stop the spam sending and to improve the reputation of their mail servers.
