The contents of header lines are interpreted only 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). The MTA normally only interprets and rewrites the outermost set of message headers. The MTA 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. The keyword noinner tells the MTA not to rewrite inner message header lines. It is the default. The keyword inner tells the MTA to parse messages and rewrite inner headers. These keywords can be applied to any channel.