553 5.1.8 <...>... Domain of sender address does not exist

Dear all,

we are using a system "consrv1" under RH 7.3 with sendmail 8.12.11
where the .cf file was installed via this .mc file

divert(0)dnl
VERSIONID(`$Id: generic-linux.mc,v 8.1 1999/09/24 22:48:05 gshapiro
Exp $')
OSTYPE(linux)dnl
DOMAIN(eso-io.com)dnl
GENERICS_DOMAIN(eso-io.com)dnl
define(SMART_HOST,mail-s)dnl
FEATURE(generics_entire_domain)dnl
FEATURE(accept_unresolvable_domains)dnl
MASQUERADE_AS(eso-io.com)dnl
MAILER(local)dnl
MAILER(smtp)dnl

mail-s is running Solaris 8 and is the DNS and MX server for the
"eso-io.com" domain.

e-mail from user 'oracle' to Rainer.Blaes@space.eads.net from the
"consrv1" system is not delivered instead this message is appearing:


   ----- The following addresses had permanent fatal errors -----

<Rainer.Blaes@space.eads.net>
    (reason: 553 5.1.8 <oracle@consrv1.eso-io.com>... Domain of sender
address oracle@consrv1.eso-io.com does not exist)

   ----- Transcript of session follows -----
... while talking to smtp-01.mgn.fr.:
                     ^^^^^^^^^^^^^^^^

>>> MAIL From:<oracle@consrv1.eso-io.com>
<<< 553 5.1.8 <oracle@consrv1.eso-io.com>... Domain of sender address
oracle@consrv1.eso-io.com does not exist
501 5.6.0 Data format error

"smtp-01.mgn.fr" is the MX mailer for the space.eads.net domain.

Although I'm using masquerading on "consrv1" i.e.

> /tryflags HS
> /try esmtp oracle@consrv1.eso-io.com
Trying header sender address oracle@consrv1.eso-io.com for mailer
esmtp
canonify           input: oracle @ consrv1 . eso-io . com
Canonify2          input: oracle < @ consrv1 . eso-io . com >
Canonify2        returns: oracle < @ consrv1 . eso-io . com . >
canonify         returns: oracle < @ consrv1 . eso-io . com . >
1                  input: oracle < @ consrv1 . eso-io . com . >
1                returns: oracle < @ consrv1 . eso-io . com . >
HdrFromSMTP        input: oracle < @ consrv1 . eso-io . com . >
PseudoToReal       input: oracle < @ consrv1 . eso-io . com . >
PseudoToReal     returns: oracle < @ consrv1 . eso-io . com . >
MasqSMTP           input: oracle < @ consrv1 . eso-io . com . >
MasqSMTP         returns: oracle < @ consrv1 . eso-io . com . >
MasqHdr            input: oracle < @ consrv1 . eso-io . com . >
MasqHdr          returns: oracle < @ eso-io . com . >
HdrFromSMTP      returns: oracle < @ eso-io . com . >
final              input: oracle < @ eso-io . com . >
final            returns: oracle @ eso-io . com
Rcode = 0, addr = oracle@eso-io.com

the mail-s mqueue is showing these entries:

               /var/spool/mqueue (2 requests)
-----Q-ID----- --Size-- -----Q-Time-----
------------Sender/Recipient-----------
i15FAhqJ025289*       5 Thu Feb  5 16:10 <oracle@consrv1.eso-io.com>
                                         <Rainer.Blaes@space.eads.net>
i15FBFqJ025292*    2071 Thu Feb  5 16:11 MAILER-DAEMON
                                         <oracle@consrv1.eso-io.com>

If masquerading is working why do I see in the sender address
the hostname consrv1 here.
 
If I'm using the command

/usr/lib/sendmail -f oracle@eso-io.com ........

the mail get delivered.

What's wrong here in our environment?

In advance many thanks for every hints!

 Friendship