It could be IE9 running in IE7 compatibility view, although I can't imagine that being "the most frequent user agent".
Like you say, this is most probably a (Windows) email client. Email clients are quite archaic in their rendering abilities, so IE7 is no surprise - in fact I thought later versions of Office (2007+) went further backwards and used the rendering engine from Word?
The first part would seem to be consistent with Outlook 2007, Outlook 2010 and Outlook 2013. However, these user agents usually identify themselves as being "MSOffice" or "Microsoft Outlook" respectively.
Outlook Express / Windows Live Mail would seem to identify themselves as "Outlook-Express".
However, this question (unanswered) on StackOverflow is asking the question of how to tell the difference between Outlook 2003 (and earlier) and a regular IE browser - since the two look very similar.
My best guess therefore is that your user agent string is from Outlook 2003 (which I imagine is still a popular version). However, the niggling problem with this is that MSIE 7 only first appeared around 2005/6. It's hard to imagine that Microsoft would have looked ahead this far! If it is Outlook 2003 then I can only assume that various updates (perhaps to IE/Trident) will have updated the UA string also?