Sendmail: Virtusertable and MX

I witnessed an interesting behavior today in sendmail. I have an entire email domain mapped to a single user in /etc/mail/virtusertable: @domain.com domainuser.

When the MX record pointing that domain to my host went away, leaving only a CNAME record aliasing domain.com to the name of my host, sendmail stopped recognizing the entry in virtusertable completely. Sendmail was checking against my local user database directly, instead of virtusertable. Maybe an A record would have made sendmail recognize it, and it was just the CNAME breaking it.