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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?

Remy Lebeau
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user1424739
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  • AFAIK, it is not generally used in practice. In the 25+ years that I've been online, I've never once seen a fragmented message spanning multiple emails. And most email providers have a cap on email sizes anyway, making fragmentation even less likely. – Remy Lebeau Jan 13 '23 at 01:00
  • I know that it is not used for fragmentation of the same message. I observe that it has been used to reference previous emails (not just the one being replied to directly) in the same thread. That is why I want to know how it is used in practice. People who are familiar with email clients might be in a better position to answer. – user1424739 Jan 15 '23 at 05:13

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