Email receipts do not generally work across all clients. Each client can turn them off, or they may be filtered by the email server. While it may be possible to rely on them in certain intranet settings, there is no way to enforce getting them over the internet.
Be aware that many people regard read receipts as an intrusion of their privacy. I would advise to consider carefully where and when to use them, and in general, just don't do it.
Ping & DNS - updated with new look and Ping home screen widget