Alias names are not freed after deletion - to be assigned to other inboxes as alias

Alias names are not freed after deletion - to be assigned to other inboxes as alias

Problem I am experiencing: 
Alias names are not freed after deletion - to be assigned to other inboxes as alias

First I had admin@mydomain.com created with N.R.Murthy@mydomain as alias for the admin also other variations of it: NR.Murthy, N.Rmurthy and plain NRMurthy @mydomain

Then I decided have a separate inbox for Mr N R Murthy, I removed all aliases and created separate for him. But when I was trying to assign other variations of his name with dots as aliases just as mentioned above, it shows an error saying, those aliases are still associated with admin@mydomain email id.

I am unable to move forward please help. If you want to know the real email id username for which this problem is occurring, I can send you a private email/message if you tell me how, sorry I got an impression of this as an open forum (unlike raising a private ticket). Please write back.

Thanks in advance.