[Exim] Exim as a secondary MX

Hi,

I am having some trouble setting up a secondary MX with Exim as the MTA.
I went through the archives and found a similar request about relaying
issues, but could not find a solution for Exim 4.24 yet.

The scenario I have is as follows:
mx1.mydomain.com (sendmail)
mx2.mydomain.com (Exim)

mx1 is a primary MX, mx2 a secondary and has mydomain.com in
/etc/secondarymx .

Now, what has actually happened is:
Emails from ordb.org written as
"marvin@??? were accepted by the secondary mx,
relayed to the primary mx, which in turn didn't deliver them locally, but
instead relayed to marvin@???, resulting in a temporary
listing at ordb.org.
My question about this is:
why did the Exim box accept "marvin@??? in the
first place ? Is that a valid address according to the RFC ?

If it is, is there a way to reject messages written like that in Exim ?
It should not relay them at all.

Regards,

Steffan