Exchange 2013 - False FQDN in Email Header

Hello,

we have an exchange 2013 cluster:

2x CAS-Server
2x Mailbox-Server (DAG)

CAS-Server 1:
smtp1.domainname.de
IP: xxx.xxx.xxx.150

CAS-Server 2:
smtp2.domainname.de
IP: xxx.xxx.xxx.151

The FQDN is correctly set on sendconnector on both cas servers.
Now we have the following problem:

We send many emails to externa mail system and then check the header on received mails.

Received: from smtp1.domainname.de (xxx.xxx.xxx.150) => That's right

or

Received: from smtp1.domainname.de (xxx.xxx.xxx.151) => That's false. We have false fqdn here. fqdn does not match with ip-adress. Correctly it must be smtp2.domainname.de

We could not find any false configuration on our side.
Do you have an idea how we can fix this problem?

Thanks