You will be in trouble if it is. So many weird thing would happen, such as can’t receive the email from some domains but not on the other, or some people can’t receive email from your while others can. Once the mail flow is going funny, it tells you it is time to check if your mail server is open relay secure and if the IP address of your mail server is on the blacklist. Check the link below, which contains a vast array of various tools that would help you to do the verification. If you are unlucky and find it is on the list, then you have to deal with the provider and try to address it out why it happens.