RPost, which specializes in registered e-mails, has invented a kind of electronic Post-it note for electronic correspondence.

The innovation comes from common confusion that arises when copying multiple recipients in an e-mail: The copied recipients sometimes don’t know why they’re being brought into the discussion, unless the sender sends a separate explanation e-mail or picks up the phone to tell them why.

Called SideNote, it’s a virtual yellow note embedded at the top of the e-mail. The note and its contents are only visible to the copied recipient.

SideNote is offered along with RPost’s main service, which is creating e-mails with the kind of delivery confirmations available through the postal service. Downloaded into Microsoft Outlook, the program stamps e-mails with proof of delivery, content and receipt so that conversations about a settlement price, a deposition or an audit can be used as legal documents. RPost’s confirmations were accredited by the U.S. Postal Service in 2002.

Bar associations across the country use RPost’s product, along with the U.S. Government Accountability Office and the Census Bureau. Over the past year, the company has doubled its client base to 500. These include the United Nations’ World Intellectual Property Organization, NYSE Euronext, Whole Foods and the Supreme Court of Louisiana.

“Registered e-mails primarily benefit the company by making e-mail conversations more secure,” said Zafar Khan, the company’s chief executive. “What we’ve found with SideNotes is that it relieves some immediate personal pain for e-mail users themselves.”

Founded in 2000, El Segundo-based RPost has doubled its employees to 50 people over the past two years. In 2008, the company expects it will grow the staff to 75.