The Header field lets you assign specific headers to be included in the digital signature. Why would you want to do this? There is evidence to suggest that some mail servers and relay systems modify email in transit. If this occurs, it is possible that the email could fail verification. When you include headers in the Header field, they cannot be altered. This provides an extra level of security.
However, the risk of alteration is low and we recommend that you refrain from choosing headers. When you do, ListManager automatically includes a single header--the Sender header--if present; otherwise, it includes the From header. This approach seems to work fine for most of our users. Some users choose to include crucial pieces of information that their systems rely on for further processing, such as the Message-ID or a marketing campaign ID code. When you assign headers, ListManager still automatically includes the Sender or From header.
The contents of the Header field in a received email can be seen in the DomainKey-Signature or DKIM-Signature headers as the "h" tag/value pair. In the example below, this is displayed as:
h=Date:Message-ID:From:To:Subject:MIME-Version:Reply-To:Content-type;