It started partly at Hack Day, Kremer and Yahoo Developer Network leader Chad Dickerson told WebProNews in a conversation ahead of the formal debut of the Yahoo Mail SOAP web service.

Attendees at that event got to see a preview of the API, leading to a nifty hack created by Leah Culver.

Details behind her hack, featuring Flickr, Greasemonkey, and Yahoo Mail have been posted as a video slideshow.

She built a Flickr Postcard, which takes the text entered in a form for the postcard, and finds an image on Flickr to match it. Then the postcard goes through the Yahoo Mail service to its recipient.

(If anyone’s curious about when the next Hack Day will be held at Yahoo, well, we don’t know either. We asked, and another one hasn’t been planned yet. Dickerson said a lot of people have been asking that question.)

Kremer clarified how developer usage of the API will differ between Yahoo Mail and Yahoo Mail Plus, the premium version of the email product.

Developers will be able to create mail preview utilities for the free version of Yahoo Mail.

Applications with richer features can be built for Yahoo Mail Plus users; those apps could be ones that list inbox contents, do scheduled email backups, or enable the user to create and send an email as in the Flickr example.

There’s a little something extra on the table for application developers.

Approved developers who refer new people to become premium Yahoo Mail Plus account holders will earn a commission of $10 per referral during a trial period.

Another slideshow available from Yahoo provides a walkthrough of the Yahoo Mail API. Ryan Kennedy, known for his role as Yahoo Mail evangelist, narrated the 12-minute presentation.

As with other Yahoo web services, developers can make their work part of Yahoo’s Application Gallery.

The broad reach of Yahoo’s mail service should encourage clever designers; a truly great mashup could end up attracting a lot of attention from that sizable email userbase.