Potential Support Issues with Host Firewalls and SparkPost IP Addresses

In setting up this plugin, I ran into the same timeout issues that many users have been experiencing. In my case, it was because the SparkPost SMTP IP addresses were not registered in my host’s firewall. SparkPost support was very slow to respond to my request for their IP address(es). When they did respond, they asked if the host can configure their firewall to use the SMTP domain name instead of the IP address because the SparkPost IP addresses are not permanent and subject to change without notice.

The Unix firewalls used by high end hosts cannot accept domain names to unblock ports. Ports can only be unblocked for targeted IP Address(es). When I explained this to SparkPost support, I received the following reply:

Here are our shared IP addresses, however, these are prone to change without notice.

1 – 54.244.48.130,
2 – 54.244.48.140,
3 – 54.244.48.142

Since host firewalls need to be configured with IP addresses. When an IP address is a moving target and subject to change at any time, configuring the firewall is at best a temporary solution. Even though your host may be extremely responsive, any disruption in the business services you provide to your customers can be disastrous. For this reason, it may be better to wait for awhile before switching to SparkPost, or at least until this plugin can be configured to use HTTP sending that will work on generic ports (80/443).

https://wordpress.org/plugins/sparkpost/