What email clients is the MageMail email template compatible with?
The MageMail responsive email templates are compatible with basically every major email client out there, outside of Outlook 2007, which is an old client that's probably more in use in corporate environments.
It's was hard to find an exact number on Outlook 2007 marketshare. This post has a nice breakdown but it's pretty dated.
This page here I think is more up to date but it doesn't distinguish Outlook 2007 from other versions of Outlook:
If Outlook is 11% of market share, and if Outlook 2007 is roughly 30% of total Outlook usage, then I'd guess it's around 3% of total market share.
At the end of the day, it comes down to what your customers are using.
What I usually recommend in dealing with email client issues is to keep an eye on the email client demographics in Mandrill to see how your customer base breaks down:
Here are screenshots of a sample email in various email clients to give you a feel for compatibility: