The idea here is a forwarder of ordinary e-mail that imposes DSR discipline. Assume a Internet site P that has light weight financial relations with each of many clients. The main traffic at the site is authenticated messages to and from clients. For each client the site keeps: Transactions between clients and P are:
Forward mail to another client
The other client is identified by the other client’s Pmail address.
Message = (Pmail address of target, worth, payload)

Worth is deducted from sender’s account and
Message = (Pmail address of sender, worth − postage, payload)
is sent to target.

Forward mail to user without account.
Ordinary e-mail is sent to target including header and the payload.
Message = (e-mail address of target, worth, payload)

The header says in a natural language that a Pmail account has been established and balance created or incremented. A account secret is included in the e-mail which can be used to gain control of the account and balance. Nascent accounts are kept for one year.

Here is money
Client pays P by some conventional payment scheme.
Client’s account is incremented.
Client says “pay me” amount.
If amount is less than client’s balance P pays client, by conventional means. Client’s balance is decremented.
Create account Message = )