Livemail keeps sending back ‘Fail to send’ messages

Hi there, can anybody help me with this annoying problem. I send invoices from my documents in excel, when I send them I get a notification from the postmaster general saying that it failed to send. I look in my sent box and see it has been received, then I have to text message my client to see if they have received it. very frustrating. I probably think I'm doing something wrong here. Thanks