You need to use PR_CONVERSATION_INDEX property.
From MSDN:
Tracking Conversations
Conversation tracking is collecting responses to a message. Clients should
set two properties that aid in tracking conversations:
PR_CONVERSATION_TOPIC
PR_CONVERSATION_INDEX
PR_CONVERSATION_TOPIC is the normalized subject of the message, the subject
without the prefix strings. Set this property to the value of the message's
PR_NORMALIZED_SUBJECT property.
PR_CONVERSATION_INDEX indicates the position of the message within a
particular conversation. It is a client's reponsibility to set
PR_CONVERSATION_INDEX for each outgoing message, whether it is a new
message, a forwarded message, or a reply. Clients can set this property
manually or call ScCreateConversationIndex, a utility function provided by
MAPI.
ScCreateConversationIndex generates the value of a conversation index for
any outgoing message. ScCreateConversationIndex implements the index as a
header block that is 22 bytes in length, followed by zero or more child
blocks each 5 bytes in length.
The header block is composed of 22 bytes, divided into three parts:
One reserved byte. Its value is 1.
Five bytes for the current system time converted to the FILETIME structure
format.
Sixteen bytes holding a GUID, or globally unique identifier.
Each child block is composed of 5 bytes, divided as follows:
One bit containing a code representing the difference between the current
time and the time stored in the header block. This bit will be 0 if the
difference is less than .02 second and greater than two years and 1 if the
difference is less than one second and greater than 56 years.
Thirty one bits containing the difference between the current time and the
time in the header block expressed in FILETIME units.This part of the child
block is produced using one of two strategies, depending on the value of the
first bit. If this bit is zero, ScCreateConversationIndex discards the high
15 bits and the low 18 bits. If this bit is one, the function discards the
high 10 bits and the low 23 bits.
Four bits containing a random number generated by calling the Win32 function
GetTickCount.
Four bits containing a sequence count that is taken from part of the random
number.
If you choose to set the conversation indexes of messages manually, consider
the following suggestions:
Keep differences in the respondents' time zones transparent; use UTC times
rather than local time.
Indent each conversation group by the same amount.
Sort responses to the same message date.
Separate threads started at different times that happen to share the same
topic.
To implement a categorized sort so that messages are grouped by topic, sort
by PR_CONVERSATION_TOPIC first and then by PR_CONVERSATION_INDEX. To present
the results of the sort, set the PR_DEPTH property to 0 for messages with a
conversation index that is 22 bytes in length. Then, for every 5-byte
increment in the length, increment the value of the PR_DEPTH property by
one.
The PR_ORIGINAL group of properties can also be used for conversation
tracking. Set these properties to link reply or forwarded messages to the
original message. All of the PR_ORIGINAL properties are optional. If you do
not explicitly set, for example, PR_ORIGINAL_AUTHOR_ENTRYID, the message
store provider can use the default value, or the value of the
PR_SENDER_ENTRYID property. Likewise, if you do not set
PR_ORIGINAL_AUTHOR_NAME or PR_ORIGINAL_SUBMIT_TIME, these properties can
default to the values of the PR_SENDER_NAME and PR_CLIENT_SUBMIT_TIME
properties, respectively.
Dmitry Streblechenko (MVP)
http://www.dimastr.com/
OutlookSpy - Outlook, CDO
and MAPI Developer Tool
