6to4 Proxy (SNAT) for SMTP / failing SPF-Check

Hi, I have a short question:

We have an F5 that is used to publish our services via IPv6.

Internally we have only IPv4 in use, so the F5 does a proxying with its own IPv4 SNAT IP.

The current problem that we face is that the admin of our Spam Gateway just told me that his SPF checks for IPv6 fail, because he is receiving the mail from a "wrong" IP.

received-spf: SoftFail (smtp1.abc.com: domain of

 transitioning testuser1234@gmail.com discourages use of 10.0.0.1 as

 permitted sender) identity=MAILFROM; client-ip=10.0.0.1;

 envelope-from=testuser1234@gmail.com; helo=mail-wm1-x32a.google.com)

Is there any way to mitigate this?

Thanks in advance!