### 12.2.2.2 Additional notes

Here are some notes on the parsing semantics:

- Most non-multipart type messages are parsed as a single message object with a string payload. These objects will return 0 for `is_multipart()`.
- One exception is for message/delivery-status type messages. Because the body of such messages consist of blocks of headers, `Parser` will create a non-multipart object containing non-multipart subobjects for each header block.
- Another exception is for message/* types (more general than message/delivery-status). These are typically message/rfc822 messages, represented as a non-multipart object containing a singleton payload which is another non-multipart `Message` instance.