Many email providers such as web hosts, ESPs and even VPS providers are familiar with the consequences of being blacklisted; angry customers calling the support because of delayed or reject email, countless of hours tracking down abusive users and patiently trying to get of the blacklists.

Unlike many other anti-spam products marketing themselves as “turn-key” solutions, Halon provides a scriptable email gateway that works as a toolbox for hosting providers. It enables them to tailor the system to fit them perfectly using our high-level scripting language. For example, you can in a programmable fashion create rate limits of anything you like. If you can identify customers based on their sender domain (enforced by the sending email server), you can defer messages based on the customer’s current deliverability statistics such as script such as

if (rate("delivery-failures", $senderdomain, 0, 3600) > 999)
    Defer("$senderdomain has more than 1000 failed deliveries during the last hour");
if (GetMailQueueMetric(["filter" => [ "senderdomain" => $senderdomain ]]) > 500)
    Defer("$senderdomain has exeeded the max queue limit of 500 messages");

Although quite different from inbound spam, filtering outbound spam can be extremely effective with the right tools, because you know who the sender is. In order to create a maintenance-free system, you can even allow a low rate of spam (per customer) sail through, to minimise the impact of false positives.

There are however many other factors that can be weighted into the equation. We have compiled a short list of the most common and effective methods to combat outbound spam which includes (but isn’t limited to);

Most of what we’ve discussed here works equally good in a fully transparent proxy installation, suitable for VPS providers that (for whatever reason) have chosen not to enforce the usage of an SMTP relay.