Short answer
HubSpot's KB: a team email channel lets your team view and reply to emails sent to a shared address. You can connect Gmail, Office 365, or set up hosted email. All emails to it are visible to users with inbox access. A team address can be used only once, and not also as an individual connected inbox.
1. Channels
Inbox channels in my demo portal.

2. Fallback
See channels and fallback email.