After a long, long time with qmail, I acknowledged the onward march of progress and acceptable licensing terms; I moved on to Postfix. I've found it offers me the balance of features that I want and need from an industrial-strength MTA, with excellent configuration flexibility.
To make your copy of Postfix drop all email into one mailbox, do the following:
Create a file called /etc/postfix/virtual, replacing username with your desired delivery target:
Next create a file called /etc/postfix/virtual_domains, with the single line:
run postmap to compile these two files,
postmap /etc/postfix/virtual postmap /etc/postfix/virtual_domains
You then need to add or change the following two lines in your /etc/postfix/main.cf
virtual_alias_maps = regexp:/etc/postfix/virtual mydestination = regexp:/etc/postfix/virtual_domains
...and all email will now be dropping into one inbox. If you need or want separate inboxes for testing purposes, the virtual_alias_maps file, /etc/postfix/virtual, is the place to look.