[exim] Problem when sending mail to nonexistent domain

Greetings,
I'm experiencing the following problem: if I send an email to e nonexistent
domain from Horde, I receive the usual mail from the Mail Delivery System
telling me the address is not correct (perfect behaviour); if I send the
same message to the same wrong address from Outlook the mail is rejected and
Outlook receives an unknown answer, raising a 800CCC0B error. If I send an
email to a valid address (domain) the mail is sent correctly.
This is a big problem since if I mistype an address then my email will not
be sent, it will stay in the Outbox and the error from Outlook doesnt help
to understand that I mistyped the address.

Any suggestion?

Thanks in advence for your help.