Hey Vinod,
Is “Do not permanently delete items until the database has been backed up” option selected in Mailbox Database Properties -> Limits tab, if yes then it wouldn’t be deleted until full backup runs.
Reference:
Mailbox Database Properties > Limits Tab
You can remove it manually with Remove-Mailbox command.
$Temp = Get-MailboxStatistics | Where{$_.DisplayName -eq “User Name”}
Remove-Mailbox -Database ”DB Name” -StoreMailboxIdentity $Temp.MailboxGuid
References:
Remove-Mailbox
Can’t see Deleted Mailbox in Disconnected Mailbox under Recipient Configuration