IMAPMessageRendererIMAPCallback could be a good name.
virtual Data * dataForIMAPPart(String * folder, IMAPPart * part); needs to return the content of the given part, which you can do using 
```
session->fetchMessageAttachmentByUID(String * folder, uint32_t uid, String * partID,
                                                   Encoding encoding, IMAPProgressCallback * progressCallback, ErrorCode * pError);
```