I do something similar with labels and filters. I basically setup multiple email addresses in my Apps email account, then create a Label for each one. Then se

I do something similar with labels and filters. I basically setup multiple email addresses in my Apps email account, then create a Label for each one. Then setup filters to check the 'to:' and 'from:' fields for a given email address, and apply the label. Now your 'Labels' sidebar acts as that drop-down that you wanted.