From ProductiveMail Technologies:
ProductiveMail lets you know in real time when your e-mails are received and read. Its main benefits are: make sure that your outgoing e-mails were received and not blocked as spam; real time notifications when an e-mail you sent is read for the first time; reminders to follow-up e-mails you are waiting for an answer or e-mails that appear lost. It informs you at all times the status of your messages: the date and time they were sent, the date and time they were read and the time elapsed between both events. You can configure the notifications and warnings that ProductiveMail will use to keep you informed about all events. You can change the text that will appear in the message footer. You can hide the message footer to make the read receipt completely invisible. Your messages are never stored or routed through our server so your privacy is guaranteed. Did your message get thru? Don't wonder.