(host relay.rzone.de[81.169.145.96] said: 450 4.1.0 Don't use the Backup MX 'relay.rzone.de' while the Primary MX is available - please send your mail to xx.xxxxxx.xx [xxx.xxx.xxx.xxx] (in reply to RCPT TO command))
budy:~ budy$ host -t mx rzone.de
rzone.de mail is handled by 10 fwmx.rzone.de.
budy:~ budy$ host fwmx.rzone.de
fwmx.rzone.de has address 81.169.144.237
fwmx.rzone.de has address 81.169.144.234
It seems like your exchange server is trying to send mail to the backup mx host - although as budy pointed out, the current DNS record for the domain only lists the primary.
I suggest you test your mx lookup on your exchange server and see what answer you get (use the following command: nslookup -querytype=mx rzone.de). If you get a different answer from budy's result, flush your dns cache and try again.
It would seem that at some point, the backup mx record was published and your server got that information. Maybe there primary server was down for a short while? Anyway, their policy is never to accept mail on the secondary mx host if the primary host is available.
The reason for this is, a lot of secondary mx hosts are simpler machines without all the antispam/antivirus protections that the main mail host has, so spammers often try to send mail to the backup hosts hoping to avoid the filters that would stop their garbage from being delivered.