SMTP-Bit - An email protocol to fight spam using Bitcoins.

Core Idea

Email spams and spoofs are a constant nuisance for all. While filters grow in sophistication every year, the fraud perpetrators are always devising new ways around it. So here is a simple proposal to put an end to spam and scam artists (or at least make it an expensive proposition). Proof-of-work (e.g. Hashcash) and Bitcoin based spam filters have been proposed before. This protocol is simply a refinement of the latter.

Email is perhaps the most pervasively used protocol on the Internet with approx 200 billion sent per day. It is unrealistic to expect the installed infrastructure supporting emails to be upgraded to a new protocol easily. Therefore any solution must work with the existing SMTP protocol and offer an option to upgrade to a newer protocol gradually.

The SMTP-Bit protocol associates wallets with email addresses to protect against spam. An email message is required to identify the wallet associated with the sender’s email, a signature to prove that the sender owns the wallet and the public key for the wallet. Before accepting an email, the server checks the public ledger for a transaction from the sender’s wallet to the recipient’s wallet (the transaction needs to transfer a minimum of 1 Satoshi). If it finds such a transaction, the email is accepted. Otherwise it is rejected. To make the verification of the transaction from the public ledger more efficient, the email message can include the transaction id or the receiver’s mail server can store a cache of transactions involving the recipient’s wallet. The transactions can be specially tagged to make them easier to locate from the public ledger.

With such a setup, the bitcoin transaction from party A to party B serves as a trust identifier. If party A uses a different email address, they can continue to communicate with party B based on the previously established trust relationship. Spam filters based on explicit trust (called permission based filters) have been tried before. ChoiceMail is one such example. The downfall of these filters has always been the need to trust a third-party with confidential information. The Bitcoin protocol with its trust-less blockchain algorithm removes this hurdle.


For such a protocol to work, we need to provide an efficient mechanism for the recipient’s wallet to be discovered. We would ideally like the registry to be publicly accessible and not under any single party’s control. The Bitcoin transaction ledger can serve as such a registry. To associate a wallet with an email address, the user could be required to tag a transaction from the wallet to any other wallet with the email address (or multiple addresses). Such a transaction could be part of the process of setting up an email account in the first place. It may seem that having a publicly accessible email address would be folly. However, if the protocol is successful at stopping spam, such a public registry would not prove detrimental and in fact could promote legitimate communication.

The protocol will face a chicken-n-egg hurdle to get going. One will not have an incentive to setup an SMTP-Bit enabled email address while others haven’t. It would be rather lonely. However, if the value proposition of not having to spam is worthwhile, it may prove to be sufficient an incentive for multitudes to join.

Spammers and Spoofers

A spammer will need to send you a Satoshi before he/she can send you an unsolicited email. This makes it somewhat expensive to send out bulk emails. Upon receiving the spam, the recipient can simply add the sender’s wallet to a black-list thereby prohibiting future spam.

Domain level wallets

The protocol can be extended to allow the association of a domain-level wallet (perhaps as a TXT record in the DNS entry). This will allow corporations to establish trust with individuals or between corporations without having to enable it on a per-email address basis.


Marketing departments wanting to send you emails would need to establish a transaction to your wallet. You can unsubscribe by simply blocking their wallet.

High Profile Emails

The SMTP-bit protocol could also open up celebrity emails to the general public. Lets say - I’m going to pick a dead, not-generally-well-known person - Geerhardus Vos wanted his email to be publicly available but he wanted to restrict people from sending him a flood of emails. He could set up a higher transaction threshold for unknown wallets and create a white-list of wallets of his known friends and contacts (folks would then want to give a celebrity their wallet address when making an acquaintance). Nothing prevents an average Joe from setting a higher threshold either - chances are folks will not be willing to pay a higher premium to email your average Joe (or Jane!).

Loss or compromise of Wallet

If your wallet is compromised, the situation is not too different from your email address being compromised. You simply have to ask others to black-list your old wallet and re-establish trust against your new wallet.

Other Considerations

When all 22 million bitcoins are issued, miners will need to be supported by transaction fees. In such a setup, 1 Satoshi may not suffice. A simple solution then is to use 2 Satoshis and pay out 1 Satoshi as the transaction fee.