Exchange 2010.
I have one user (only one) who gets the error message, "The message was rejected by the server." when replying to messages from his iPhone 6s. He can send messages fine. He can also send and received without a problem from his notebook.
I've added his account to another I phone and the same error is received for his account, but other accounts don't have a problem sending via the Exchange (activesync) 2010 server.
Updates from LarryView