Spam is cheap because somebody else pays the bill.
Not necessarily in money.
Sometimes the payment is five seconds of attention.
Sometimes it is a moderator reviewing a queue, an abuse team tracing headers, a mail provider operating filtering infrastructure, or a business employee digging a real customer inquiry out from under fifty fake SEO pitches.
The economics have been understood for a long time. An OECD paper on spam described what it called a transfer of cost: sending bulk email is extremely cheap, while receiving, storing, downloading, filtering, and handling it imposes costs on ISPs, businesses, and individuals. See Spam Issues in Developing Countries.
The technology has changed since 2005.
The cost-shifting logic has not.
The sender buys scale; everybody else buys defense
Consider one unwanted message.
The sender pays almost nothing for the additional delivery.
The recipient has to identify it, ignore it, delete it, unsubscribe, report it, or wonder whether it is legitimate.
The mailbox provider had to accept or reject the connection, run reputation checks, inspect the message, store it if accepted, classify it, expose reporting controls, aggregate complaints, and maintain the systems that make all of that work.
The website owner receiving contact-form spam pays with moderation time.
The forum administrator pays with anti-abuse plugins and account review.
The legitimate bulk sender pays because other customers poisoned the reputation of shared infrastructure.
Each interruption is tiny.
Industrial scale turns tiny into infrastructure.
Defensive systems are themselves a cost of spam
Blocklists, machine-learning filters, feedback loops, authentication standards, abuse desks, rate limits, reputation dashboards, and moderation tools are useful technologies.
They also exist because open communication channels attract abuse.
Spamhaus says its blocklists reduce email infrastructure costs and human-resource requirements by helping administrators reject abusive traffic earlier. See the Spamhaus Blocklist documentation.
That is a benefit of filtering.
It is also evidence of the burden being filtered.
Attention is the least visible invoice
Recipient cost is particularly difficult to measure.
Five seconds deleting one junk message is negligible.
Five seconds multiplied across millions of people is not.
The same is true of false positives. Stronger filtering saves time until it blocks a wanted invoice, password reset, or customer inquiry and somebody has to investigate.
Spam Empires survive because the sender does not have to compensate everyone who handles the unwanted traffic downstream.
The campaign can fail for 99.999% of recipients and still impose work on nearly all of them.
That is the economic trick.
The sender pays to send.
The internet pays to make sending tolerable.