[exim] MX records with IP addresses

Hello,

I know that it isn't RFC conform, but we have a problem to send emails
to a Chinese ISP which MX record looks like this:

example.cn.         900     IN      MX      20 1.2.3.4.

Exim bounces a mail with the following message:

It appears that the DNS operator for example.cn has installed an invalid
MX record with an IP address instead of a domain name on the right hand
side.

What can I do to make exim deliver mails to this domain anyway?

Regards
Marten