In order to use the ecommerce services of Canada Post, a connection to sellonline.canadapost.ca or cybervente.postescanada.ca on TCP port 30000 should be established.
The IP of sellonline.canadapost.ca/cybervente.postescanada.ca is 188.8.131.52.
You should ensure that outgoing TCP port 30000 is not blocked by your hosting provider.
If you are not sure or have problems with your host check our Ecommerce Web Hosting. SiteGround has made sure this requirement is applied by default, just as everything else for your hassle-free ecommerce hosting.