I don't think it's anything to do with your computer because emails don't get sent directly from one computer to another. They get sent to your mail server which is in the place your mail company are based. When you log in to your email account if there are any mails in the server addressed to you, they then get sent to your computer. If he gets a response from the Daemon messenger it means it hasn't even been sent to the mail server. Make sure he hasn't accidentally been blocked from sending you emails and if he hasn't then it's a problem with your mail server rejecting his address for some reason.