Setup Office 365 as your mail client in Insightly (solved!)
After struggling setting up Office 365 as my mail client in Insightly (User Settings -> Email Accounts -> Office 365) I finally found a solution:
1. Install the Insightly Sidebar add-in to Outlook
2. Write a new email (from Outlook) and open the Insightly sidebar when you are typing your new email
3. This action opens a pop-up window asking for your password to grant Insightly access to your mail account. Enter your Outlook mail password (not your Office 365 password).
Once linked up, Insightly now shows the Office 365 connection when opening the User Settings -> Email account.