Why one of my msn a/c can not send messages but can receive?

I have two msn accounts, one of them works well, but the other account acts up. I can receive message through it, but when I send out message, the other side just can not receive it. And it also can not show my friends' online status. I use the same computer to login these two accounts seperately, the first account is okay, but the latter one doesn't work, as I described. I guess probably there is something wrong with the settings of the account, but I have no idea where. Could someone tell me why and how I can fix it? Thanks.