Much has been written in the last few weeks about big changes to the free e-mail offerings from Google and Microsoft. Hotmail rolled out Wave 4, with Sweep and Time-Traveling Filters that penalize “gray mail” by sweeping it out of the inbox. Gmail’s Priority Inbox now rewards messages that it deems most relevant to the recipient with preferred positioning in the inbox.

In the meantime, AOL and Yahoo! have been working quietly on their own major inbox releases. However, early glimpses at new features hint at altogether different strategies for capturing and holding users’ attention – important changes that may influence how recipients interact with your e-mail and help propagate your message.

It’s been about 20 years since AOL and e-mail became nearly synonymous (“You’ve got mail!”). Today, comScore ranks AOL mail dead last in market share behind Yahoo!, MSN and Google. AOL will attempt to rise from the ashes when they take the wraps off “Project Pheonix” later this year. Few details about specific changes to the user interface have been confirmed, but insiders say we should expect tighter integration with AOL Instant Messenger, SMS messaging and MapQuest. They’re planning a Gmail-like archive feature and enhanced search across users’ e-mail folders. AOL say they will present fewer ads at entry points, but will serve up more targeted advertising when users drill down into AOL’s content sites.

A recent interview with AOL Mail Ops President Brad Garlinghouse also hints at a larger effort to deliver an integrated messaging platform for all of the users’ various inboxes – be they traditional e-mail inboxes at Yahoo!, Gmail or MSN, as well as IM, SMS or social media inboxes. He notes that Internet users now manage an average of 2.4 e-mail addresses, up from 1.9 five years ago. Garlinghouse intimates that Phoenix will position the AOL mail site as a hub for users to manage them all.

Yahoo! has been far less cagey in discussing new features in its upcoming mail release, dubbed “Minty”. Users will be able to update their Facebook status from within their Yahoo! inbox. Earlier this year, Yahoo! made a few other social media moves, announcing partnerships with Twitter and social media game giant Zynga, and the acquisition of location-based social network Koprol.

But the most noticeable changes will be to the user interface. Yahoo! is promising a leaner, faster-loading site that more closely resembles the mobile apps (read: iPad) versions of its offerings. The interface changes are designed to increase market share in non-US markets, where consumer broadband is not as common, and load times can still be an issue for larger percentages of the user base.

It doesn’t appear that either AOL or Yahoo! are planning any major changes to the way it filters or delivers inbound mail in these releases. The message for senders is that social media and e-mail are converging quickly: Google’s abortive attempt to socialize e-mail with Buzz did not spell the end of other efforts, and it’s possible that more than one will succeed. If that happens, senders may have a better chance to turn their recipients’ entire social networks into prospects.