Breaking in late September, digital activity will promote the firm's business-mail services. Creative will complement work by its above-the-line agency, Abbott Mead Vickers BBDO.
It comes as Royal Mail gears up for deregulation of the UK postal market, which will come into effect in April 2006.
The online element, valued at £200,000, will feature different executions, depending on the nature of the web site on which they are placed.
For example, on travel sites, a piece of mail will float on to the ad from the left-hand side of the page and turn into an aeroplane. On sports sites, it will turn into a rugby ball and pass between a set of goalposts.
The push was created by Proximity London, with media planning and buying by Carat Digital.
Steve Green, interactive manager at Royal Mail, said: "It's a branding campaign, although we are looking for people to click through, where we can give them further information."
The firm also aims to push its research database, which can show businesses the power of mail when it is combined with other media, for example.
Green added: "We are getting across that Royal Mail delivers to every address in the country - we have that reach. It's about giving businesses the power to keep in touch with customers."