RPost on Monday announced its e-mail registering service will be integrated with Postini’s messaging security hosted service.

As per the agreement, Postini’s 35,000 customer sites can immediately take advantage of RPost’s proof-of-delivery service as part of Postini’s collection of service offerings, according to RPost officials. RPost provides users with proof of an e-mail’s content, receipt, and delivery date, which holds up as admissible evidence in court, officials say.

Sending a message with RPost’s Registered E-mail service is much like sending registered mail, offering proof that an e-mail was sent, when it was sent, who the recipient was, and what the original content of the e-mail was. In June, RPost added encryption, PDF conversion, data-leak protection and the ability to generate electronic contracts to the service.

Postini customers can take advantage of the RPost option by typing “(R)” in the e-mail subject line, triggering the service to register the message. Senders can choose to mark the outbound message as registered or not.

The message is then relayed from Postini’s servers to RPost’s servers, where a copy is sent both to the sender’s receipt file that’s automatically created in the in-box as well as to the recipient. The service records the server-to-server dialog to prove the intended person received the message, and any subsequent messages that are forwarded or replied to, and associates that data with the original receipt.

For Postini customers the first 10 messages sent using RPost are free. Beyond that, the cost is 59 cents per message.

Last month Google announced plans to acquire Postini for $625 million.