On Mon, Mar 15, 2021 at 12:23:54PM +0100, Robert Ionescu wrote: > I already found the callbacks for the verification process and I am > still trying to figure it out if it is possible to change them in a > way that they will print some certificate information to determine > which certificate was used? What do you mean "change them"? These are callbacks, you register the callback function in the application, and then do whatever you want in that function, including print certificate information, if that's your goal. There's nothing to "change". The verification Postfix uses for optional certificate verification verbosity is at: https://github.com/vdukhovni/postfix/blob/master/postfix/src/tls/tls_verify.c#L139-L185 -- Viktor.