Hello Alexander,

I assume you mean the class System.Web.Mail.MailMessage
(http://msdn2.microsoft.com/en-us/library/system.web.mail.mailmessage.aspx).
This class is marked as obsolete and you shouldn't really use it.
Nevertheless, the documentation for the Fields property
(http://msdn2.microsoft.com/en-us/library/system.web.mail.mailmessage.fields.aspx)
has this information: The Fields property is used to set extended
properties for Collaboration Data Objects (CDO). For more information
about CDO, see "Collaboration Data Objects" and "Configuration CoClass" in
the MSDN library at http://msdn.microsoft.com/library.
Searching for the recommended terms in MSDN brings up loads of pages that
might be useful, and the page for "Configuration CoClass" actually has a
list: http://msdn2.microsoft.com/en-us/library/ms870485.aspx

Now, let me repeat that all these pages point out you should be using the
System.Net.Mail.MailMessage class instead, and as far as I can see, this
class doesn't need any fields in this confusing format.

               Oliver Sturm