Email receipts generally don't work outside your mail service's domain.
Be warned--there is a growing resentment in the Land of email receipts. I was mildly annoyed by them back in the day when they were a part of AOL mail. Now that I am older and wiser, I strongly resent them. They serve no useful purpose except to indicate that you email was probably delivered. This is really not much in doubt anymore. Presumeably, you want to know that your recipient read and understood your message. Well, a receipt can't really tell you that.
Word to the wise: If you want to know if your recipient received your message, call him. This is why God gave Man the cellphone.