Save the message you wish to attach as an eml file, then attach the eml file as you would any other file.

Thank you Stans, was it so easy? /hans peter

You could also drag messages from the Threads Pane (message list) and drop them on the new message window (reply, forward or new) to the right of the addressing area, as with any other attachment.

Thank you, sfhowes, that is even easier. I was missing a choice in the dropdown menu. That's why I asked. /hans peter