...is header-based. You might run into issues sending threaded replies to Outlook users programatically because the headers it uses for this are nonstandard. RFC 5322 defines a much simpler, but more limited, standard for reply threading.

The Thread-Topic header is self explanatory

The Thread-Index header is base64-encoded binary data which contains a header struct + 1 struct per additional reply to the thread

Header struct format:

A Windows FILETIME is an unsigned 64 bit integer counting the number of 100ns intervals since 00:00 1 January 1601 UTC

Reply struct format:

To populate the reply struct, first we need to subtract the root email FILETIME from the reply FILETIME to produce the time difference. Then (using big-endian bit numbering here):

The delta code is set to 0 if no bit more significant than bit 48 is set. Otherwise it is set to 1. This corresponds to time differences greater than about 1.7 years having a delta code of 1.

If the delta code is set to 0, right-shift the time difference by 18 bits and store the low 31 bits of the result as the time delta.

If the delta code is set to 1, right-shift the time difference by 23 bits and store the low 31 bits of the result as the time delta

This allows us to store a larger range of time differences at the expense of worse precision for time differences larger than 1.7 years.