In Unified View mode, Canary Mail uses color marks along the right edge of the Thread List to help you quickly identify which email account each message is associated with. This feature can be especially helpful if you have multiple email accounts and want to keep track of your emails in a single inbox.
Each email account in Canary Mail is assigned a unique color, which is displayed as a vertical marker next to the message in the Thread List. When you receive a new message in any of your email accounts, the color marker for that account will be displayed next to the message. This can make it easy to quickly scan your inbox and identify which messages are associated with which account on your Windows device.