tcpdump host your.smtp.server.address port 25

in the console of your server and making an attempt to send email.
Console output should show communication with mail server.

 "id": 10197698,
  "rootId": 0,
  "code": 22,
  "name": "SYS_SMTP_FAILURE",
  "timestamp": 1622718652,
  "originTimestamp": 1622718652,
  "origin": 0,
  "source": 58849,
  "zone": 0,
  "dci": 0,
  "severity": 1,
  "message": "Unable to send e-mail to xxxxxxx@xxxx.it: SMTP conversation failure",
  "lastAlarmKey": "",
  "lastAlarmMessage": "",
  "tags": null,
  "parameters": [
      {
        "name": "",
        "value": "3"
      },
      {
        "name": "",
        "value": "SMTP conversation failure"
      },
      {
        "name": "",
        "value": "xxxxxx@xxxxxxxx.it"
      },
      {
        "name": "",
        "value": "NetXMS_A6: 47.906N.PIT.A-S01: Node down"
      }
  ]
Any ideas? The SMTP server it's ok, I've already tested it.

Thanks

Bob