I have a little VPS that is used as platform for POP'ing and SMTP relaying for internal projects and testing.  Originally it was all setup under a single domain and I had turned on masquerading.  However I needed to add another domain for a different project.

I was pretty sure I needed to use the "genericstable" to get what I wanted done.  I setup Outlook Express to send test emails through, but the "FROM" line kept getting written to user@originaldomain.com.  The reply-to line had the correct domain though.  

Well, the solution wasn't adding MORE info to the config lines in sendmail.mc, but to REMOVE everything.  Remove the masquerade, generics, and local_domain directives and use the "genericstable" to do all the "from" address re-writing just like "virtusertable" does all the "to" address re-writing.