Yahoo is letting web programmers get their hands on the core code for its e-mail program.
The initiative could mean new ways to deal with mail messages
By opening up the code Yahoo hopes to help create a series of innovative ways to deal with e-mail messages.
Eventually it said there could be "tens of thousands" of add-ons and extras for the e-mail reader.
Yahoo is only the latest of the big web companies to open up their main services to the wider code developing community.
Although Yahoo plans to keep control of the code that deals with user names and passwords, programmers will be able to tinker with almost every other part of the e-mail reading program. Yahoo Mail has more than 257 million registered accounts.
The initiative could result in new looks for the Yahoo e-mail reader, innovative ways to display messages to get a quick idea of message contents or a series of add-ons that do more with the information found in mail messages.
"Yahoo is a very large company but we can't build every application that a user might want," Chad Dickerson, head of Yahoo's software developer relations, told the Reuters news agency.
In making this move, Yahoo is following the lead of many web firms including Google, Amazon, Flickr and business software maker Salesforce.com.
Before now only Yahoo's broadband partners, such as BT, have had access to the innards of the e-mail program.