Fully Disclosed Service - the recipient is always informed - every email sent using this service includes a signature that tells the recipient that the sender keeps track of this message will be informed. This is key in order to create a new level of trust between senders and recipients that in turn will reduce the burden from recipients towards senders.
A blocker (enabling recipients to "hide") is available for those not yet ready for the change (and don't want others to know whether they opened their email messages).
Privacy as top priority - email messages never go through the service or any special servers, there is no need to modify email addresses of recipients or any change in the manner MS Outlook is used. Needless to say that this way, the content of the emails, their subject line and even the address of the recipients are not exposed to the operator of the service.
Seamless integration into Outlook - An improved "sent items" folder serves as a snapshot status-report of all outgoing mail, this folder contains the "Mailinfo reports" with the full log of where and when each message was viewed.
Real time alerts to your desktop - A real-time alert is displayed whenever a message is viewed. In addition, it will display reminders to follow up on those messages that were not yet viewed.
Reciprocity (as captured so vividly by Yogi Berra) is key - if you want to use Mailinfo you should accommodate others using it as well - that is why a mailinfo user can not use Mailinfo and at the same time be "invisible" to other Mailinfo users.
Innovative "rules of the road" philosophy - http://www.mailinfo.com/web/philosophy.htm
Best of all - its Free - well, at least the beta is...