Six months ago I stopped using Outlook and switched to Gmail. The great about that is that I can drag and drop messages in my brain. This is possible with Outlook too, but there are risks of loosing the links, for example after a reinstall of outlook (see other threads).
The downside of Gmail _was_ that contact pages did not have a fixed URL. Since a couple of weeks Google updated the contact manager in Gmail, and now they have fixed URLs per contact. Now you can drag and drop your contacts in PB without any fear of loosing the links.
Not using TheBrain anymore. Switched to Roam Research.