March 2014 when I released my fourth in series 30% of my sales were from Mobile devices.
By August of 2014 when I released Warriors 57% of my sales were from Mobile devices ... and that has held steady through my recent release. (I get the data through my affiliate links).
I know we've talked here about certain reliable advertisers not working as well anymore. I think it's because too many emails are unappealing on phones.
... I realize coding for things like Android are a b!Tch and a half and is practically its own skill set, but there are things you can have your developers do quickly that could pay off big in the end--like upping the font to 15 pixels (you can do it conditionally or across the board). Instead of putting entire blurbs you can have authors craft short snippets so that there isn't a lot of scrolling on a small device.
Manybooks.net has a BEAUTIFUL email that shows up great on phones. It makes my heart happy. I don't want to name and shame any others, but I hope they see themselves here and think about these numbers.