Is it possible for an email to not arrive...?

I sent an email to someone in my address book and he said that he did not receive it , though the address was the same one I have used before and also since then with no problems. The message appeared in my sent folder and I did not receive a mail error email from Yahoo. He had no reason to pretend he did not receive it as it was an offer of work for him as he is a plumber and he does many jobs for my company. So what happened to my email..?