Should an outgoing spam mail be detected by the filter, is there a return value / error code for the e-mail delivery? If yes, how is the SMTP error code?

Yes, an error message is issued by the filter indicating the blocked mail. As an error message you would get e. g. "550 The content of this message looked like spam". More information about message classification can be found in our FAQ article "What are the SMTP responses and what do they mean?".