A couple of things, Alex:

• If you're in an environment where the MUA (and it's config, in many cases) is a matter of compony policy, you're probably OK relying on MUA confirmations. If the people at the co. have any choice in the matter, I wouldn't rely on them soely, if at all. Provide some other way to say "I've read this".
• It's easy to get a message flagged as "read" in your MUA accidently; this would normaly send a return recipt. For example, in outlook, all you need to do is delete the prior mail and walk away. After a message has been previewed for a certian amount of time (configurable), it's considered read, even if the user wasn't actualy there. The machine has no way to tell if it's seat is occupied.
• If there's a legal reason behind the confirmation requirement, this won't fill it.
• Why not get an account here and stay a while?