I'm going crazy. Read these two lines about tha same message
This is from syslog
Nov 03 06:15:20 mx postfix/postscreen: NOQUEUE: reject: RCPT from [126.96.36.199]:64428: 550 5.7.1 Service unavailable; client [188.8.131.52] blocked using b.barracudacentral.org...
same problem for me. I'm using no licensed version for testing purpose before to buy.
These are the info required
root@mx:~# pmgversion -v
proxmox-mailgateway-container: 6.1-1 (API: 6.1-3/1975aa1f, running kernel: 5.0.21-5-pve)