Campaign Monitor, one of the best email marketing software platforms around, released some really useful and interesting email client stats today. More than 300 million uniques is a pretty solid sample size:

It continues to blow us away just how quickly the iPhone is moving mobile email forward. The iPhone now caters for 5.78% of the email client market, breezing past Gmail to become the 5th most popular email client in the world.

Do you test on the iPhone? Another interesting bit:

We figured it was a safe assumption that more and more people would be moving from traditional desktop email clients to web-based email clients. While it’s certainly clear many of us are shifting to reading our emails on a mobile device, the same can’t be said for web-based email clients. The market share for both Yahoo! Mail and Hotmail has more or less plateaued over the last six months.

Outlook’s share faltered a bit, with 2007 getting a modest gain, but 2000/2004 falling.