This issue can be related to mail filtering rules. It's possible someone from the server has sent mass or spam emails and email provider has blacklisted the IP for some time. Our admins are constantly working on this type of issues to get it resolved as soon as possible. However, please mind that the IP might be listed again at any time.
# Shared hosting servers (including Resellers)
With shared and reseller hosting, all email is routed through the IP addresses shared by the entire server so changing the IP is not a solution. We recommend upgrading to a VPS (Virtual Private Server) which will provide dedicated resources, your own dedicated IP addresses, and your own instance of control panel with no adjacent web sites.
A VPS may use one of their allotted dedicated IP addresses for sending mail. However, we will not replace a blacklisted IP with a clean IP address. This option will only be effective up to the maximum number of IP addresses that you can get for your server. Please note that it is in your control to keep the server safe and actively work with blacklists to request de-listing if it does happen.