If you send and receive your emails using a mail client, for example Outlook, you could potentially face the problem that the IP address you are attempting to send your emails from has been blacklisted. The IP is contained within the email's header and due to the use of dynamic IP addresses from ISPs, the IP you are using could be blacklisted. This could perhaps be because another customer on the same ISP as you has been bulk emailing. You may not be able to send emails.
You should receive a bounce back email from the recipients mail server informing you of this. This email will usually contain instructions to get the IP delisted. The other option would be to contact your ISP for assistance.
Please get in touch with our support team if you require further assistance.