I have been very involved in creating HTML email campaigns as of late and very interested in optimizing for the most popular email clients . . . one thing I noticed is that mobile email is on the rise as more and more people use their mobile devices (smartphones) to check email.
Campaign Monitor has a very good blog that keeps me up to date with statistics on how/where people read their email.
Read about the prolific rise of mobile email – http://bit.ly/rdmxEM
Get the updated stats on Email client popularity for 2011 – http://bit.ly/pk9LWX
