That is definitely odd and we want to make sure that you're able to receive all your messages. To be sure, what model of phone are you using? Also when you say the "@" symbol is in place of the message is it for the entire message or the number? How long has this been going on exactly? Have there been any changes with the device since this started such as new apps, app updates or software updates? Does this only happening with this one contact?
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It sounds like a problem on his end. You might consider deleting his contact and then re-adding it. He should do the same with your info on his phone.
Is your friend on Sprint by chance? This came up for several contacts I had who were using Sprint. Borked messages going to any other carrier. Sprit sorted it out after a couple weeks, but it was all on Sprint's end.