How to Navigate your Inbox
Select your inbox from the left side menu under "overview".
You will not be able to see all your messages that you have received from vendors.
You can also make some adjustments like:
Adding PM to Chat - you can add someone else from your organization into a certain inbox chat with a vendor
Mute Chat - you can turn off all notifications for a chat
Close Chat - you can close a chat after you finish chatting with a vendor. This will move it from the active tab into closed to keep your inbox messages organized.