don't see in GUI disabled users
This seems to be a very common complaint from the field - Deleted mailboxes will appear in disconnected mailbox list,
but these changes it will not reflect immediately. You have to wait for online maintenance to run and complete before the mailboxes are correctly listed
open Exchange shell and type:
for all database:
Get-Mailboxdatabase | Clean-MailboxDatabase
or for single database:
Clean-MailboxDatabase MyDatabase