Just a heads-up type post.
Basically, the problem I've encountered is that Sitefinity extensively modifies the original html-css of email-campaigns, presumably to ensure they display on the lowest level of html email client, and offers no obvious way to override its default actions.
Whilst this may be a solid engineering approach, by making sure that the html will work on all email clients, it actually results in html emails that are a poor fit for the intended purpose of the module - email-marketing campaigns.
Research says that well in excess of 50% (now nearly 60%) of all emails are read on mobile devices, and research also says that emails read on mobile devices are a one-shot opportunity - i.e. The user who can't easily read an email on their iPhone will NOT then attempt to read it on their desktop/laptop - they will simply delete it, and any marketing opportunity is lost.
The mobile device market is obviously dominated by iOS and Android, and virtually all native email clients on these two platforms are html5/css3 compliant and support media queries (even Outlook on these two platforms does this) offering email marketers the opportunity to provide easily readable and engaging content, without needing to re-size, zoom, rotate and perform all other manner of girations, just to read them.
It's entirely possible to construct html emails that will work optimally on all email clients - responsive-mobile, web-browser, outlook-desktop, etc, etc - although admitedly it does require a bit of research, a bit of effort, and a good test-preview platform, such as Litmus - although, just as a side-comment, wouldn't true client previews be a killer feature for Sitefinity to have - but I digress...
The bottom line is that Sitefinity will not currently support the creation of responsive html emails or Outlook-specific emails (i.e. Conditional comments etc) and this has a massive impact on its usefulness for email-marketing... so much so that my own marketing people insisted that we move our email-marketing to CampaignMonitor, and test with Litmus... more money for us and more headaches for me.
Whilst I'd actually prefer an "advanced-options" UI in the campain template management (because I'm basically lazy) if the ability to register a code-behind on a template allowed access to override the API(s) which currently rewrite emails before they are rendered to the smtpsender, and stop Sitefinity from doing some of its default actions, then it may be possible to create responsive html emails and emails that adjust for Outlook (using conditional comments) etc etc... although I haven't yet actually gotten to the bottom of what might be achievable.
Anyway, tech-support created this feature request for me (thank you) so if it's of interest to you, feel free to vote for it.