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MailChannels Outbound Filtering uses a combination of content analysis, domain and URL reputation checks, and behavioral signals to detect spam. Despite this, no filtering system catches 100% of spam without ever blocking legitimate mail. Here’s why.

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The fundamental trade-off

Spam filtering is a classification problem: every message is either spam or not spam, and the filter must decide. The challenge is that the cost of each type of error is very different:
  • False negative (spam delivered) — the recipient receives an unwanted message. Annoying, but recoverable.
  • False positive (legitimate mail blocked) — a message someone needed never arrives. This can cause real harm — a missed invoice, a lost job opportunity, a failed account verification.
Because false positives are so damaging, MailChannels tunes its system to minimize them, even at the cost of letting some spam through. A filter aggressive enough to block every piece of spam would inevitably block too much legitimate mail.

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Why spammers have the advantage

When a spam campaign is blocked, the spammer can simply adjust the message and try again. The cost of sending another attempt is nearly zero. For the filter, however, every tuning change carries the risk of introducing new false positives. This asymmetry means spam filters must be conservative.

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What MailChannels does to maximize accuracy

  • Content similarity matching — compares messages against known spam campaigns
  • URL and domain reputation — checks links against blocklists of known malicious domains
  • Behavioral analysis — monitors dozens of sending patterns across all accounts to detect abuse signals

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What to do if spam gets through

If you see spam being delivered from MailChannels IP addresses, report it. Forwarding the message helps improve detection for the entire platform. See Report false positives and false negatives for instructions.