Connection Error: 421 Service not Available, connection refused

Sometimes mails are not sent and remain in the outgoing messages queue.
The above error message can be seen in the protocol when trying to deliver these mails.
What does this mean?

The destination device is either temporarily unavailable or there is absolutely no mail server available for this mail.
This is typical for a spammer.
It is also possible that the DNS of the destination device is not configured correctly.
Make sure that the Recipient Check on the REDDOXX appliance functions correctly.
If not, the mail will be bounced by your own mail server.
Normally, the spammer uses a false sender address, therefore the mail cannot be delivered.
It is important that your mail server is not programmed to send a "Non-delivery report" to a spammer, and activating the Recipient Check on your appliance will prevent this.
Also check the Filter Settings in the Spamfinder.
If the "Action" of a Negative Filter is set to DECLINE, a bounce mail will be generated.
REDDOXX Support recommends setting the "Action" to QUARANTINE in the Filter Settings.