Windows 7 PC with groupwise remote client directly interfacing to external non-GW POP3/SMTP server.
I first found this in gw2014r2 (previous versions not checked), and now in gw18beta. It is a mail header issue which breaks conversation threading. To reproduce it,

1. Send whatever mail to A@HisProvider
It goes with a Message-ID: <5ACDA5D2.B918.0000.1@MyProvider>

2. A receives mail with header
In-Reply-To: <5ACDA5D2.B918.0000.1@MyProvider>
References: <5ACDA5D2.B918.0000.1@MyProvider>
Message-ID: <216e0135da3e5456f048a2aceb0b39fc@HisProvider>
This is OK.

3. A replies to you with whatever client (Web or POP, but non-GW)
You get mail with new Mesage-ID and correct thread references (as above).

3. Reply to A on that mail
The sent message header (checked on My and His side) is something like
References: 0001e69e4607a1b3 !!!
In-Reply-To: 0001e69e4607a1b3 !!!
Message-ID: <5ACF8F4F.B918.0000.1@MyProvider>
Instead of using the Message-ID of the generating thread, groupwise inserts arbitrary 16-digit-numbers (which vary in position 5-8 on different replies, but are completely unrelated).
This is not OK.

4. A's mail handling is dependent on conversation threading. So he complains that my mail-replies always open a new thread.

Does anybody have an idea, except for using another mailer for thread-sensitive communication?
Thx for any enlightenment, EN.