Most of my emails are not being received by our customers. I don't send out newsletters, so it's not being marked as spam. I send out quotes only. Most of our customers will ring and ask why they haven't received their quote. I'm not having trouble sending the emails and quite often get a delivery receipt but it's not actually getting delivered. In this case I have no other option but to print it out and snail mail it to them. This is affecting our business as we can't win any work if customers are not receiving our emails. Why are my emails not being received?