iOS Mail
Apple Mail for iOS is one of the most popular email clients on the market. Shipping with every iOS device (iPhone, iPad, and iPod Touch), iOS Mail collectively represents a staggering 36% of the email market. Email designers should feel comfortable designing for iOS Mail, though. Like its desktop counterpart, iOS Mail is a first-class email client with excellent rendering capabilities.
iOS Mail has similar capabilities as the desktop version of Mail and has out-of-the-box support for popular email platforms - including Gmail, Yahoo Mail, Outlook.com, AOL Mail, and Microsoft’s Exchange server.
Market Share
Just like iOS devices themselves, Mail enjoys incredible popularity. While we differentiate between iPhone and iPad in our breakdown - collectively, iOS Mail has 36% of the market cornered - more than any other platform or client available today.
As of November 2013, Mail on the iPhone held the #1 position in our email client charts with 24% market share, while Mail on the iPad came in at #3 with 12% market share.
Deliverability
Similar to the desktop version of Mail, iOS Mail relies on the service it uses to send and receive mail for spam filtering. iOS Mail does not have any advanced settings for junk mail rules, but it does allow users the ability to move messages to a junk folder - affecting the deliverability of future messages for that user only. This type of spam filtering is similar to many email clients that have user-specific, learnable spam filtering.
Rendering

iOS Mail is not only one of the best mobile email clients, but also one of the best email clients overall. It has rendering capabilities greater than many desktop and webmail clients and readily supports most HTML and CSS. Mail also supports many advanced design techniques including the use of most CSS3 properties, SVG, and CSS animations.
However, iOS Mail is not without issues. iOS devices will automatically resize text that is set at a font-size less than 13px, which can lead to broken email designs. This can be remedied by including the following CSS:
-webkit-text-size-adjust:none;

A more common rendering issue is the display of dates, times, phone numbers, or addresses as blue links regardless of whether or not the designer wrapped them in an anchor tag. We cover the best solutions for dealing with blue links over on our blog.
So long as you keep those two issues in mind, most designers will find iOS Mail a joy to work with and can spend the time saved here fixing issues in Outlook.