The contents of header lines are only interpreted when necessary. However, MIME messages can contain multiple sets of message headers as a result of the ability to imbed messages within messages (message/RFC822). IMTA normally only interprets and rewrites the outermost set of message headers. IMTA can optionally be told to apply header rewriting to inner headers within the message as well.
This behavior is controlled by the use of the noinner and inner keywords. noinner tells IMTA not to rewrite inner message header lines. It is the default. inner tells IMTA to parse messages and rewrite inner headers. These keywords can be applied to any channel.