After alot of testing on my system I came to know that
after disabling a mailbox this mailbox will be displayed in "Disconnected Mailbox" in Exchange Management Console only if this mailbox is accessed once ( ie. user logged in to that mailbox atleast once). All the mailboxes which are just created and haven't been logged in atleast once , will not be displayed in "Disconnected Mailbox" in EMC after being Disabled (either via Disable button on EMC or via Disable-mailbox Cmdlet in PowerShell).
Worse is that if we Disable a newly created mailbox (which has not been logged in (accessed) atleast once ) then we can't Enable that mailbox again via Connect-mailbox Cmdlet (Nor this mailbox is displayed in EMC as I mentioned above). This way that disabled mailbox is lost permanently.
Now I have two quries about it
1. Is there a way to programatically view all the disconnected mailboxes ?
2. Is there a way to programatically log in to a mailbox ( bcoz Exchange doesn't create a mailbox in mailbox store untill this mailbox is logged in).