Fortimail S/MIME certificate binding error

Hi,

I'm trying to test S/MIME encryption with our Fortimail VM (6.0.3).

But I am not able to create a certificate binding profile for the public key of an external test account.

I've tried several different certificates, but the fortigate doesn't accept any of them.

Every time I get the error "Certificate binding: invalid certificate or password".

Can anyone tell me in which exact format the Fortigate accepts certificates?

Thanks in advance,

Boris