Over the last several months, we have seen many customers adopting Microsoft Teams, even if their mailboxes are still hosted in an on-premises environment. One of the common issues in this scenario i...
Updated Nov 09, 2023
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boutzamat - My understanding of the license requirements is all users must be licensed for Teams and their identity needs to be synced via Azure AD Connect to O365 to support on prem integration.
Microsoft does require an Exchange Online license where compliance/retention requirements come into play for Teams 1:1 Chat data as this is stored in an online mailbox created per user (commonly called a shadow mailbox). If you end up migrated on prem to O365 the contents of this shadow mailbox are merged into the inbound mailbox.
All of my customers that are deploying Teams Calendar integration are on an E3/E5 license so there is no cost to enable these services or move mailboxes to O365, it's purely a business decision on where they want to host mail. I have only deployed calendar integration for on-prem Exchange as a coexistence/interop while migrating mailboxes to O365.