Would you take a moment to elaborate? I know it's possible using HTML 5 but I'm specifically curious about older specifications such as
!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
CampaignMonitor has two versions: agency and non-agency. Non-agency is great for any business managing their own email marketing. The agency version allows you to manage multiple clients or departments from a single account and set per-client billing options, customize the interface to match your brand, and markup/resell it to your clients. If you can prune your list and get it to 5,000 or below you would qualify for the $49/mo plan. Otherwise the next plan up for 5,300 users is $99/mo.
I don't think you'll be disappointed. Most of the application is forms-based so it's hard for even the most novice users to screw up.
I would definitely recommend CampaignMonitor. I've used it since 2007 and it has one of the easiest and most intuitive dashboards you'll find among the popular ESPs. The reporting is straightforward and simple to understand even for the most basic user. CampaignMonitor is a white-label application and it started with designers/developers in mind, so it's perfect for rebranding and marking up the pricing to earn a little passive income.
I haven't personally used Sendgrid yet but I saw their dashboard and was very impressed with how intuitive it is. There seems to be a lot more granular-level features and reporting than CampaignMonitor and they offer a free plan.