If you are using the BH DNS(ns1.bluehost...), then check to see that the $from address is an existing email in your BH account. (go to your CP /domain manager and use whois info to find that out.) or..... maybe, you migrated the source code, but did you transfer your domain name to BH, or is it pointing to BH servers (DNS) In that case your site is not using the BH mail servers. example :your site content could be on BH, but if it is registered somewhere else and the DNS is not ns1.bluehost.com..etc, then your domain might be using url forwarding or framing (to bring visitors to the BH location). but your mail functions may be handled by the registrar's
hope this is not too confusing it is also helpfull if you post your domain name here and others can help figure out as well.