I have been using https://efa-project.org/ in front of my ISPCFG3 for year. Basically all email domains hosted on this ISPCFG server have their MX set to point to EFA which filters and then forwards emails to ISPCFG3. I now have the problem, that some spammers seem to be cleverer than me and directly send their SPAM to my ISPCFG bypassing my EFA. Can someone give me pointers how to allow my ISPCFG server to receive emails only from my EFA? Also, if none of my MX records points to ISPCFG there shouldn't be a problem with this setup, right?