Do you have issues with user's mailboxes somehow not getting removed from Central eventhough you have deleted the AD user from the Active Directory and synchronized with ADsync tool successfully?
This is an expected behavior as per the documentation in the link below:
Here is the screenshot of the statement:
So basically, the behavior is that if nothing else is connected to the user or mailbox in Central, then as soon as you synchronize the account it will be removed. However, if anything is connected to it like for example an Self Service Portal login, an endpoint device, administrator role, etc. then the account will not be removed.
What to do
Delete the user manually from the 'People' page (this will remove the mailbox for that user as well within the 'Mailboxes' page)
After 4 hours, the license slot should be relinquished.
Special consideration: Please make sure that the user being deleted is not connected to a Sophos Mobile account as this can affect its operation. In these cases, please get a case created with Sophos Support so an alternative procedure can be done.