Per RFC 2046:
The inclusion of a "References" field in the headers of the second and subsequent pieces of a fragmented message that references the Message-Id on the previous piece may be of benefit to mail readers that understand and track references. However, the generation of such "References" fields is entirely optional.
I see this. It does not explicitly mention how it is usually used in practice. Does anybody have a summary of common email clients and online email service providers (e.g., Gmail) on how this is implemented in practice?