To obtain BodyStructure from IMAP you need to use Imap.GetBodyStructureByUID method directly:
Imap imap = ...
List<long> uids = imap.GetAll();
List<BodyStructure> structures = imap.GetBodyStructureByUID(uids);
...or you can use Imap.GetMessageInfoByUID to and navigate MessageInfo.BodyStructure property.
Imap imap = ...
List<long> uids = imap.GetAll();
List<MessageInfo> infos = imap.GetMessageInfoByUID(uids);
BodyStructure first = infos.SingleOrDefault().BodyStructure;