OT: sendmail with Exim secondary MX

[Steffan Henke]

Sorry for being OT this time, but I got a question regarding
both sendmail+Exim so I hope to find a guru here instead of being flamed
by either sendmail or Exim people on their list.

The scenario I have is as follows:
mx1.mydomain.com (sendmail+MS)
mx2.mydomain.com (Exim, no MS yet, I am not really used to Exim yet)

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 at marvin.ordb.org"@mydomain.com were accepted by the secondary mx,
relayed to the primary mx, which in turn didn't deliver it locally, but
instead relayed to marvin at marvin.ordb.org, resulting in a temporary
listing at ordb.org.
My question about this is:
why did the Exim box accept "marvin at marvin.ordb.org"@mydomain.com in the
first place ? Is that a valid address according to the RFC ?

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

Again sorry for being OT, I would appreciate help via PM if you mind
replying to the list. If there is any general interest, I can summarize
though.
Meanwhile, I removed the setup and the secondary host is not listed
anymore, but the whole thing buggers me, especially because I reject
emails at the SMTP level if the host is listed at ordb.org, duh.

Regards,

Steffan