[exim] customer certificate problems in to TLS communication…

Hi

I've another problem with TLS and a customer certificate.
I've a exim-4.44 with openssl support.
The customer has registered the certificate with a second level CA.
When I recive a mail from the customer server, TLS communication isn't
estabilished andinto mainlog I found the messagge "unknown ca".
The customer has provided his own certifiate to be installed in our
exim.
The certificate file has a extention .p7b, it seems to be in pkcs7
format.
I need to configure exim in order to accept the certificate in the TLS
communications.
How make it? I'm reading openssl and exim documents but I don't
understand what I need to do.

Andrea