abstract:User Data Header (UDH) is a binary structure which may be present at the start of a short message in the Short Message Service in GSM. It does not contain any text, but it specifies how the message should be formatted and processed.
As Apple is using the reply-to address of a message's UserDataHeader to identify the origin rather than the raw source, receiving iPhone owners risk being fooled by a phishing attack (or just a dishonest acquaintance) that poses as a contact or a company.