Brafton has reported that quality, custom content is a must for email marketing, and a new blog from Yahoo Mail makes it increasingly clear that marketing messages must be relevant. Yahoo engineers explain in a blog post that the Yahoo Mail app works even during readers' idle time to ensure that messages are prioritized according to users' interests.

Whenever users take time to read messages or news stories in their inboxes, the Yahoo Mail application sets to work configuring "new data and functionality that you may use in the future." Key among these updates is the app's mission of downloading messages according to the likelihood that users will read them.

This function seems similar to the Gmail Priority Inbox or Microsoft’s email features that declutter inboxes, which Brafton reported necessitate relevant email marketing messages. Still, Yahoo seems to think its work during very brief idle times sets this inbox apart.

"Instead of blindly downloading your entire inbox like some other email providers do, we anticipate the messages that you will read," frontend engineer Subramanyan Murali says.

To ensure marketing messages are read by Yahoo users, marketers must offer content they will be interested in. From industry-related news headlines to targeted promotional offers, brands might find that investing in content that will catch clicks and merit Yahoo Mail-preferred downloads pays off.

Marketers should take note of this relevancy check, because Yahoo Mail is one of the web's most popular email destinations. By Hitwise's measure, Yahoo Mail was the third-most visited website for the week ending January 15.