But the platform itself has not changed much in that time.  Most email inboxes today look more or less the same way they did 10 years ago.  Specifically, they follow a "last in-on top" hierarchy (though users have long had the ability to sort in other ways) and they mix all the classes of email together.  At any given moment you can look into your inbox and have a note from a friend next to a CNN news alert next to a free shipping offer from J. Crew.  And that doesn't even take into account the corporate stuff.

And while this design was not necessarily intentional, it had a huge benefit for marketers.  A user going to the inbox to check for email from friends and family will stumble across some (hopefully) great offers from marketers.

But now we are starting to see shifts in the way large mailbox providers construct the inbox experience, mostly in response to the loss of eyeball share from social networking sites -- where an increasingly large share of person-to-person messaging is occurring.  Dubbed "the social inbox" by industry pundits, these innovations focus on including social networking features (like status updates) within the inbox and also make it easier to find and respond to messages from friends and family.

The most recent foray into the trend came from Yahoo, which unveiled changes to its mail interface.  The new UI features a "What's new" tab that is displayed by default when you log in.  This tab shows unread email from contacts (meaning any email address in your Yahoo address book) and connections (meaning people you are connected to through Yahoo).  And, as part of an earlier update, the inbox also gives users filtering options for viewing email.  You can now view "all" email, email just from contacts, or email just from connections. 

Meanwhile, the "What's New" tab includes other social media features like status updates from connections and contacts (people in the address book) from Flicker, Twitter, YouTube, Facebook and more.  You can read more about these changes -- and see screen shots -- at the Yahoo mail blog.

What should email marketer do in the wake of these changes?  I see three key takeaways:

Change is the new constant:  Mailbox UIs are probably going to change more frequently. There will be more innovation as Yahoo continues to iterate, and other large mailbox providers try their version of the social inbox.  You are going to have to learn to adapt -- quickly.

Engagement has a whole new meaning:  Being a "contact" is more important than it used to be at Yahoo.  In the past, "add to address book" has largely been a deliverability tactic.  It now becomes more important at Yahoo.  Perhaps it will become more important at other mailbox providers.   What are you going to do to make it worth the subscriber's while to add you as a contact?  What value are you creating?

Friends with benefits:  In addition to being in the address book, it may be that certain types of "friending" or "following" or "connecting" will also be important.  Again, the same question: What can you offer Yahoo subscribers and others to make it worth their while to friend you or make you a connection?  This also creates interesting opportunities to cross social media marketing with email marketing.  For example, Yahoo's "What's New" tab shows tweets from connections before the messages in the inbox.