Database design for web mail client
I think it has to do with your security requirements / permissions on the backend.  With the single table approach, DB permissions will be for the table, whereas you can permission the multiple tables per user.

You can partition your single inbox table by user. This will allow you to manage it in much the same way as you would the multiple tables for performance.

If you decide on separate inbox tables per user, I would recommend putting them all in their own dynamically created filegroup.

What will happen to the tables or rows when the user leaves?  In two scenarios, 1 where they are dropped and 2 where there are archived, both would be more easily and cleanly accomplished by the table scenario IMHO.