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 given by the filter indicating the blocked mail. For example, you would receive the error message "550 The content of this message looked like spam" if the filter recognized the outgoing message as spam. You can find more information about the classification of messages in our FAQ article "What are the SMTP responses and what do they mean?".