My, how reading email on handheld devices has grown – something we need to increasingly bear in mind with newsletter sends and email marketing campaigns.

According to Litmus and based on data gathered from a billion email opens, smartphone and tablet opens in the last 6 months have increased by a whopping 80%.

While there has been more than double the number of opens on Android operating systems compared to the company’s 2011 figures, the iPhone still rules the roost (for now) in terms of handheld opens.

Opens in Outlook have been particularly affected as a result – down 51%.

Here’s how the market share statistics look:

iPhone – 20%
Outlook – 18%
Yahoo Mail – 13%
Apple Mail – 9%
Hotmail – 8%
iPad – 8%
Android – 7%
Gmail – 5%
Non-specific webmail – 5%
Windows Live mail – 3%
Other – 3%
AOL mail – 1%

More from Litmus here.

So if you’re not already doing it (and I’m only rarely doing so), testing how a campaign or newsletter looks on an iPhone every time you run a mailout would be a wise move.

Testing for display and other issues is such an important part of email marketing. It’s the times I haven’t that are the times a problem will occur and once a message has gone out, that’s it. It’s a horrible feeling to see a glaring error in a campaign, knowing that thousands of people will also see it.

Before each send these days I test how a message looks in Outlook, Gmail, Yahoo and Hotmail (and going forward, iPhone). If you can get another set of eyes to look over the same tests, it certainly helps as “store-blindness” can be an issue if you’re the only one reviewing.

The Outlook testing isn’t just about display either. I send to two different accounts on two different services which I retrieve via Outlook to examine spam scoring, which can be found in the email header (the non-visible coding) assuming the ISP is using a filter, which most do now.

Learn more about testing email campaigns against spam filters.