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The terms SnowShoe Spammers is well explained on the Spamaus Site in this article about snowshoe spam.

Like a snowshoe spreads the load of a traveler across a wide area of snow, snowshoe spamming uses many frequently-changing IP addresses, domains and aliases to spread out the spam load in order to dilute recipient reputation metrics and evade filters. Snowshoers use many fictitious business names (DBAs), fake names and identities, and frequently changing postal dropboxes and voicemail drops. Conversely, legitimate mailers try hard to build their brand reputation based on a real business address, a known domain and a small permanent range of sending IPs. Snowshoers often use anonymized or unidentifiable whois records, whereas legitimate senders are proud to provide their bona fide identity.

Some showshoers use tunneled connections from their back-end spam cannon to the spam egress IP. The back-end IP address is not in the spam headers. ISPs, you are in a position to detect those back-end spam cannons by checking where traffic flows are coming from. Remember, the tunneled connection is not necessarily on port 25. Spamhaus always appreciates such information.

Here is a documented sample of snowshoe spam relating specific IP addresses, mail server names and domain names with email addresses that relayed spam to our clients in the past 3 days.  This is a quick sample to illustrate the snowshoe spam with specific examples.