IEmailThreadMetadataThreadIndex Property

Composed of a base ID, plus a unique number for each message within the thread, and if there are attachments, the letter A plus the unique number for the attachment within the thread. For example, A1, A2, A3 Uses the format: [base ID].[unique number for message].A[unique attachment number]. The base ID is the same as the thread ID without the leading zeros. The thread index for the root message of the thread will just be this. ex: 4.1.2.3.4.5.6.7

string ThreadIndex { get; }