[sylpheed:37224] Manually grouping unthreaded messages

Hi Sylpheed users,

I sometimes receive messages from correspondents which have the same
subject line and are part of the same chain, but for whatever reason
(perhaps the other user's mail client configuration) are not treated as
belonging to the same thread by Sylpheed on my end. I have "Thread
View" enabled, and other messages are threading fine, so it does seem
that this is particular issue with only certain mails.

For easier viewing, I would like to link these messages so that they
belong to the same thread. Is there a way that I can manually do this?

Many thanks.

    - Ben