What is WHM?

[villavic]

Mentions in multiple places, no clarification of what that is.

https://github.com/ibm-cloud-docs/email-delivery/blob/master/how-can-i-setup-alternate-port-smtp-whm.md https://github.com/ibm-cloud-docs/email-delivery/blob/master/index.md https://github.com/ibm-cloud-docs/email-delivery/blob/master/getting-started.md https://github.com/ibm-cloud-docs/email-delivery/blob/master/FAQs.md