Sophos Central Email Gateway S/MIME valid signed emails changed/untrusted

We have moved one customer to Sophos Central Email Gateway.

Since then all valid signed emails arrive at the customer's server as untrustworthy.

Sophos Central Email Gateway apparently makes changes to the emails in such a way that it invalidates the signature.

At first we thought it was because all URLs in emails are being changed.

But even when deleting the URL from the footer, the emails still arrive as untrusted.

Also when we deactivate the option to change URL in valid signed emails the URLs still get changed. The option seems therefore not to be working.

HowSigned Emaildo we prevent valid signed emails from beeing invalidated.