I need a help for sending messages! I cant sent my mails to the ones who use Gmail (just Gmail).?

I need a help for sending messages! I cant sent my mails to the ones who use Gmail (just Gmail). Everytimes after sending the mails, I receive this: "Hi. This is the qmail-send program at yahoo.com. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out." This problem is just for Gmail!!!