Yahoo has developed an improved search technology for its webmail service, including the ability to search the full text of message attachments, the company plans to announce today.

The new search system will enhance the existing capabilities, which are very basic and only allow users to search for content in a message's subject and sender lines and its body, a Yahoo executive said.

Starting today, the improved search function will be rolled out in phases over the coming months to users worldwide, said Drew Garcia, a senior product manager on the Yahoo Mail team.

Yahoo Mail's new search function can index the full text of about 20 common types of attachments, including Microsoft Word, Excel and PowerPoint files, as well as Adobe PDF files.

Another improvement is that, along with each search result, Yahoo Mail also delivers a snippet of the content in which the searched-for term appears. This is intended to give users an idea of context.

Users don't have to do anything to enable the new search function. Yahoo will automatically index, in the background, all existing content of each Yahoo Mail account, as well as all future messages received, once the search function is enabled for a user, Garcia said.

The new search function applies to Yahoo Mail messages stored on Yahoo's servers, not locally on users' hard drives. However, Garcia said Yahoo is likely to integrate it into other products and services, possibly including its desktop search software.

Another new Yahoo Mail search feature is the ability to see thumbnail images of pictures and documents attached to email messages. Users can also open the actual documents from the thumbnail images.

The new search matches the first letters of search terms, something which comes in handy when the user isn't sure of the correct spelling of a word. For example, a user could search for "rob" and the system will return matches that begin with those letters, such as "robbie" and "robin".

Yahoo Mail's new search function will also let users refine searches and drill down on results to help them find the information they are looking for faster.

Users will be able to view photos or attached documents in a list of search results by themselves, in thumbnail form, Garcia said. Each thumbnail will be labelled with the subject line of the associated message.

The new search capabilities were developed in-house by Yahoo and prompted by the realisation that users are increasingly storing a wide variety of information in their email accounts, making it necessary to provide them with more robust search capabilities, Garcia said. The search system was developed from the ground up to work in an email setting.

The improved search will be part of a major revamp that Yahoo Mail will undergo as Yahoo incorporates technology it acquired when it bought Oddpost in July 2004 into the webmail service, a Yahoo spokeswoman said.

The fruits of that overhaul, such as a significantly redesigned user interface that looks and feels more like a desktop email client than like a webmail interface, will begin to appear when Yahoo starts an open beta test of the upgraded Yahoo Mail in the coming months, she said.