While looking for sender_login_maps I stumbled accross this thread. Sorry if I might seem hijacking this...
Why are you using MySQL directly here? In /etc/postfix/imscp I have lists for aliases and mailboxes. Putting these together should give us a good senderlist.
Actually this was how I had used the aliases in ispcp. There the aliases file contained mailboxes as an alias referring to itself like this:
So well... I'd either go and check everything via SQL (aliases, mailboxes, ...) or have imscp generate map files. So just wondering...