'Retrieve the structure of the body of one or more messages
This command is allowed in the Selected state.
@type messages: C{MessageSet} or L{str}
@param messages: The messages for which to retrieve body structure
data.
@type uid: C{bool}
@param uid: Indicates whether the message sequence set is of message
numbers or of unique message IDs.
@rtype: C{Deferred}
@return: A deferred whose callback is invoked with a dict mapping
message numbers to body structure data, or whose errback is invoked
if there is an error.  Body structure data describes the MIME-IMB
format of a message and consists of a sequence of mime type, mime
subtype, parameters, content id, description, encoding, and size.
The fields following the size field are variable: if the mime
type/subtype is message/rfc822, the contained message\'s envelope
information, body structure data, and number of lines of text; if
the mime type is text, the number of lines of text.  Extension fields
may also be included; if present, they are: the MD5 hash of the body,
body disposition, body language.'

def fetchBodyStructure(self, messages, uid=0):

    return self._fetch(messages, useUID=uid, bodystructure=1)
