The GetHeader method returns the value of a header field
from the specified message. This allows an application to be able to
easily determine the value of a header such as the sender, or the
subject of the message. Any header field, including non-standard
extensions, may be returned by this method.
This method uses the XTND XLST command, which is an extension to
the POP3 protocol. If this command is not supported by the server,
the method will attempt to retrieve the entire message header and
return the value for the specified header field. This enables an
application to use this method even if the server does not support
command extensions.