received mail from old host name and new one

I am running RedHat ES 3.0
I have configured my machine to just send outgoing emails to a relay server.
I have shutdown sendmail since I do not need it.

Here is the problem..
I changed host names, and now I get dupicated emails, one using the old host
name and one from the new.

AC Switch care <(E-Mail Removed)>

AC Switch care <username@oldname>

Couple of questions...

I never stipulated anywhere that the "from" should be AC Switch care, where
does it pick this up from...

And second, the 2nd email just has the hostname and not hostname.domain,
where do I correct this?