Effective September 2020, Microsoft 365 F1 does not include rights to a Microsoft Exchange mailbox. In order to enable a full Microsoft Teams experience, Microsoft 365 F1 licenses may come with an additional Microsoft Exchange Online Kiosk (K1) license. Although the Microsoft Exchange Online K1 license will provision a mailbox for the user, it is important to notice, that Microsoft 365 F1 users are not entitled to use the mailbox. Microsoft recommends disabling Microsoft Outlook on the web via these steps: https://docs.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-user-mailboxes/enable-or-disable-outlook-web-app and asking affected users not to access the Microsoft Exchange mailbox via any other methods.
For further information on Microsoft 365 F1 User Mailbox Use Rights, please find the Microsoft Product Terms here: https://www.microsoftvolumelicensing.com/Downloader.aspx?documenttype=PT&lang=English.