This is especially true if you are the kind of person who saves most of his or her email. That mound of messages can be a treasure trove of contact information and a history of your interactions with hundreds, or thousands, of personal and business acquaintances. It can tell you the phone numbers and job titles of people, and even who you and your correspondents most often copy on email. It’s a sort of social network all its own.

The trouble is, it’s hard to tease all that information out of the typical email program. And that goes double for the most popular, but most bloated and dense, email program of all, Microsoft Outlook (MSFT).

Now, however, there’s a new, free plug-in module for Outlook that adds a set of social-networking and data-mining features right inside the venerable program. This new plug-in for Outlook is called Xobni, which is “inbox” spelled backward and is pronounced “ZOB-nee.” It is completely contained in a colorful vertical panel that lives on the right side of your Outlook screen and doesn’t block or intrude upon Outlook’s own panes or functions.

I’ve been testing Xobni and I like it. The product has some flaws, and is still a work in progress, but I found that it made Outlook much faster and more useful. Xobni turned my Outlook experience from one that was organized by messages and dates into one that was organized by people, relationships and histories.

Xobni, available at Xobni.com, works with the 2003 and 2007 versions of Outlook, whether you are running Windows XP or Windows Vista. The San Francisco start-up behind Xobni, also called Xobni, is working on versions for other email readers, such as Yahoo Mail (YHOO).

Xobni indexes all your stored email, starting with the most recent messages. Once the email is indexed, the Xobni sidebar shows a profile of the sender of any email you have selected in the message list in your inbox or other folders. Each profile is divided into useful sections, and as you drill down into the specifics on each person, the entire sidebar may fill with more detailed information.

At the top of the Xobni panel is a huge search box. This in itself makes it worth installing the program. It is much faster and better than Outlook’s own search, returning results almost instantly and in two lists — people and mail — with the search terms highlighted.

But that’s just the start. Under the search box is the name of the sender of any email you have selected, with a photo, if available, and a bar chart showing how many emails you’ve received from that person and the times of day when that person appears to be most or least active on email. The latter can be a handy guide to deciding the best time to send an email for a quick response.

Under that is a section carrying the person’s phone number, extracted either from Outlook’s contacts or from the person’s email signature or the body of his or her emails. If Xobni can’t find a phone number, you can click on a link that appears where the phone number would have been, to generate an email asking for a number.

Similarly, Xobni can generate an email asking for a meeting with any person in its profile and fill in that email with your open times for the next few days, drawn from your Outlook calendar. You can also open a new, blank email form, preaddressed to the person in the profile, with a single click.

If the person has a profile on the business-oriented LinkedIn social-networking service, the Xobni panel will display his or her job title, employer, and a photo from LinkedIn.

The final three sections in the Xobni sidebar are even more useful. The first one shows the person’s email “network” — a list of everyone that person has placed in the “To” or “Cc” fields of emails you’ve exchanged. The next section is called “Conversations” and displays the entire threads of all the email exchanges you’ve had with the profiled person. Finally, there’s a section listing all the files you have exchanged as attachments with the profiled person.

Each of these sections has its own search, and allows you to open the entries it contains.

There are some bugs. In my tests, Xobni mistakenly merged the profiles of a few people. It also failed to recognize that the emails bearing different versions of a person’s name — like “Thomas” and “Tommy” — but with the same email address, were the same person and should have a single profile. Xobni also can cause glitches during manual syncing of BlackBerrys and other devices. The company says it is working on fixing those bugs.

But, overall, Xobni is a smart addition to Outlook that makes email much more useful.

• Find all of Walt Mossberg’s columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com. Email him at mossberg@wsj.com.

The result is a polished, fairly powerful email program that I prefer to Google’s much-hyped Gmail, which is undergoing an even longer gestation. It has been in beta status since April 2004.

I’ve been testing the new Yahoo Mail on both Windows and Macintosh computers. It has some downsides, but it beats Gmail, in my view, both in terms of features and in terms of its ability to act like a standard computer program rather than a Web page, something for which Gmail often gets more credit.

A closer competitor to Yahoo Mail is actually Microsoft’s Hotmail, now called Windows Live Hotmail. But Yahoo tops Hotmail, too, in my opinion.

The new Yahoo Mail, which works in Internet Explorer and Firefox on Windows, and in Firefox on the Macintosh, is now more than just an email program. Like Gmail, but unlike Hotmail, it has a built-in instant-messaging module. You can choose to communicate with any of your contacts via a real-time chat, right from within Yahoo Mail, as long as that contact is online and has an IM account on either the Yahoo or Microsoft instant-messaging networks. You don’t need to be running your IM program.

Unlike either of its competitors, however, the new Yahoo Mail also allows you to exchange text messages with people on cellphones, although the message exchange must be initiated from Yahoo Mail.

Yahoo Mail offers unlimited storage of emails and attachments free of charge, and a very fast and good search capability — like Gmail’s — so you can keep years of messages on hand and retrieve them quickly. Gmail offers 2.9 gigabytes of storage free. It sells extra storage for prices ranging from $20 a year for six gigabytes to $500 a year for 250 gigabytes. Hotmail is in the process of boosting its storage to five gigabytes, free, and 10 gigabytes for $20 a year.

With Yahoo Mail, you can send attachments of up to 10 megabytes per message and 20 megabytes if you opt for a $20-a-year plan that also eliminates the annoying banner ads that litter the free version. Gmail offers attachments of up to 20 megabytes free. Hotmail allows 10 megabyte attachments and 20 megabytes under its $20-a-year plan, which also banishes ads. Gmail has no banner ads, just text ads that run alongside the emails and can’t be eliminated.

This new Yahoo Mail is gradually being rolled out in coming weeks. The company still plans to retain the older version of Yahoo Mail, now called Classic, for people who prefer it, or for those using browsers that are incompatible with the new version, such as Apple’s Safari.

The new Yahoo Mail allows you to do things that once were impossible in a Web-based email program. For instance, you can drag messages to new folders, or select a group of messages in the same way you would with a standard email program, to delete them or mark them as read or unread. Unlike in Gmail, when you right-click on a message you get a list of options that pertain to the mail program — like “Reply to Sender” — instead of options that pertain to the use of the browser — like “Add to Favorites.”

Also, Yahoo Mail features a very nice tabbed interface that Gmail and Hotmail lack. With this interface, which is separate from the browser’s own tabs, you could have your inbox in one tab, an instant-message or text-message conversation going on in another and a new email you are composing occupying yet another. You can move among these tabs without losing the content in any of them.

And like Hotmail but not Gmail, Yahoo Mail offers a preview pane, like Microsoft’s Outlook, so you can see the contents of an email without opening it. Gmail offers just a “snippet” of the message content. Unlike Gmail, which forces you to view your emails as bunched-up “conversations,” Yahoo Mail — like Hotmail — displays them as a standard email program does, sorting them by date, sender, subject or size.

So what are the downsides of Yahoo Mail?

Well, the biggest is probably that unless you pay for the $20-a-year premium plan, you can’t view your Yahoo mail account in a standard email program such as Outlook or Apple Mail. Gmail allows this free of charge. Hotmail allows it free in Outlook and in Windows Mail, though it will soon announce the capability for other email programs for premium members and, eventually, for free members as well.

Also, I found Yahoo Mail could sometimes be slow. When I created a new folder and tried to drag 200 emails into it, I was warned that I couldn’t do that because the new folder was “still being created.”

But overall, Yahoo did a really good job making its online mail program versatile, powerful and accessible.

Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.