As per version 6.6.153 (and 6.7 if you are using the testing branch), you can now use drag and drop to attach e-mail messages to other e-mails, as well as images and other files.
Although the e-mail module is one of the older modules of Group-Office, we occasionally add new nifty features. Due to a generous donation, we could free the resources to add this and make our e-mail module yet more powerful and user-friendly.
Instead of writing down a long story, we decided to upload a nifty little video. In the undying words of one of our funniest comedians: the video lasts only ten-ish seconds, so you'll be bored less than a minute.
A demonstration of how to drag and drop anything