Veeam Backup for Microsoft 365 can only backup mailboxes that are licensed*. If a mailbox is selected to backup and does not have a license assigned, Veeam will report a warning stating the “Mailbox does not have a valid Microsoft 365 license.” There are two options for resolving the “Mailbox does not have a valid Microsoft 365 license” warning.
Option 1: Assign a valid license to the mailbox that you would like to backup.
Option 2: If the mailbox does not need to be backed up, then you can exclude the mailbox. Below are the steps to exclude a mailbox:
If you are backing up your entire organization:
- Navigate to the Select objects to exclude tab.
- Click the Add button to open the users list, select the user that is experiencing the warning, and select click Add.
- Select the user and Click the Edit button. Check the Mail box to exclude, hit OK, and save the job settings.
If you have specific objects selected to backup:
- Navigate to the Select objects to back up tab.
- Select the user from the list and click the Edit button. Uncheck Site to exclude, hit OK, and save the job settings.
* Shared and resource mailboxes are the exception, and do not require a license to backup.
Source: https://www.managecast.com/veeam-for-microsoft-365-mailbox-does-not-have-a-valid-microsoft-365-license-warnings/