I have an anti-spam system based on 'postfix'. When someone sends an email and is blocked by a blacklist, they get feedback.
Example below:
This is the mail system at host mail.xxx.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<barbara@doris.herbert.lab.go4labs.net>: host
doris.herbert.lab.go4labs.net[37.59.133.20] said: 535 5.7.8 Error: blocked
by Block Address check from xx.xx.xx.xx (in reply to RCPT TO command)
Where can I disable 535 error notifications?