Major Description
We don't need to generate a MimeDescriptorStructure if a client issue the following commands:
FETCH 1 BODY[....]
FETCH 1 BODY.PEEK[....]
The generation of such a MimeDescriptorStructure can be really heavy weight as it may need to parse the whole message content. So its better to skip the generation of it whenever possible.
Its only needed for the following commands:
FETCH 1 BODY
FETCH 1 BODYSTRUCTURE