No service avaliable

I am attempting to use a custom SMTP email server (running Mail-in-a-box) to send email from, but keep running in to this error. When I do the connectivity test for my custom SMTP email server, it says that there is no service available on any of the ports listed (25, 465, 587). Firewall shouldn’t be an issue, as I have this set up on multiple other devices, such as my iPhone, iPad, and Mac. I have triple checked the hostname and it is correct, and I have also checked the settings in my PHP and WordPress config files, and they are both correct. I am at a loss here, because I have these exact same settings set on multiple other devices and they all seem to work fine.

https://wordpress.org/plugins/postman-smtp/