1. Include /etc/mail/virtusertable as a source of valid users.
2. I don't know how many other admins change this, but I have on my personal server. I have Sendmail configured to use multiple aliases files so that can segregate system/user aliases, Mailman aliases, and my website staff aliases in three separate files. More things to edit, but easier to find what I'm looking for. It would be nice if, instead of hard-coding /etc/aliases into the script, you parsed /etc/mail/sendmail.cf and extracted the value of the "O AliasFile" line.