Since Teams requires a corresponding Exchange mailbox to function, with F1 license the users would still get an Exchange mailbox.
The difference is that with F1 license they cannot directly access the mailbox, while they can access the mailbox with other licenses like E5,F3.
For more details please refer to this documentation: How Exchange and Microsoft Teams interact
If you are using an external mailbox instead of the Exchange mailbox, there are some features like calendar events which may not be available.
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