How internet mail threads work in Domino 8
The new mail conversations views in Notes 8 can work with both Notes native email and Internet mail. The latter requires a Domino 8 mail server to process the mail. Here is the slightly geeky explanation:-
Domino 8 added support for ‘References’ and ‘In-Reply-To’ IETF RFC 2822 header fields in addition to the ‘Message-ID’ header. The sending email system would also have to support these header fields for conversation threads to remain intact.
For Domino 8, we did some limited testing with various popular internet email systems. (this is not intended as support statement). At the time, only some of these mail systems appear to support both new header fields and not all behaved consistently. Even Microsoft Outlook appeared to have different support levels for these headers when using different protocols - eg MAPI vs IMAP and POP. Google Gmail and Thunderbird IMAP did appear to support the headers in our tests.
So internet conversation threads will work depending on the other system you are collaborating with.

Blog