<firstname.lastname@example.org>: host mx.destination.com [188.8.131.52] said: 552 5.2.0</email@example.com></div> Invalid 7bit DATA (in reply to end of DATA command)The 552 5.2.0 error code led me finally to the solution which is the IP address where the email is being sent from is blacklisted. Steps to confirm:
1. Navigate to www.myip.pk to find out the machine's public IP address
2. Lookup the IP at the www.mxtoolbox.com blacklist database
3. Most likely your IP will be listed there. This means spam was sent from that network, usually infected machines can cause this.