Guest post written by Miriam Geller of Yahoo!.

What do Yahoo! Mail and a chameleon have in common?

Both are changeable with separately mobile stereoscopic eyes and rapidly extruding tongues.   Okay, scratch the eyes and tongue thing, but both are definitely changeable.

Once upon a time, when you sent an email, its content was forever cast in stone.  That meant lost opportunity for senders and frustration for customers receiving your message.   Frustration because the great Valentines Day deal email opened on Presidents Day, is no longer valid.  Lost opportunity because while your customer is lamenting the fact that they missed the V-day deal, they aren’t opening your Presidents Day message that contains a different discount they can use today.

What if the email message you sent could change like the stereoscopic-eyed chameleon?  The Valentines Day message, if opened after V-day, could show information about a different, still redeemable discount.  The one-day restaurant deal, when opened, could automatically let the customer know that the deal is expired and show a currently redeemable deal instead.   These emails would be more effective for the sender and much more satisfying for the customer to receive.

Now, you can deliver dynamic, timely email messages just like this with the Yahoo! Mail Development Platform.  The platform empowers you to create applications that can:

• dynamically change how messages are rendered.
• inject javascript into an email to make it interactive.
• call external web services to expand or update an email’s content.
• launch an app-specific UI within the Yahoo! Mail chrome.
• create new ways to compose, read, and organize messages.

How do Yahoo! Mail Users get your applications?

There are a several ways a Yahoo! Mail user gets their hands on your app:

1. Contextually. Users experience your application as part of their regular mail flow.  The email message comes in, your app is applied against it and voila, your recipient sees today’s deal not yesterday’s.
2. Via the application list on the lower left hand side of Yahoo! Mail.  Installed applications that aren’t strictly contextual get an icon in the application list.  Yahoo! Mail comes preinstalled with applications from Yahoo! and third-party developers that let you:
   1. Easily and securely send files up to 100MB per email.
   2. Organize messages — newsletters, coupons, promotions, receipts, etc into folders — leaving email from real people in your Inbox.
   3. Edit, enhance, organize, and share photos directly from within Yahoo! Mail.
3. Via the application gallery.  Yahoo! Mail users can install applications that didn’t come preinstalled.

Come meet us at Inbox Love.  We’ll be going deep at the Yahoo! Mail tech track and we’ll also be around to discuss all things Inbox at our post event cocktail table. In the meantime, go to http://developer.yahoo.com/mailapplications/ to get started today or contact us at developer-questions@yahoo-inc.com. See you there.