How do I get the current host within the context of an automation email?

Normally you can use {{context.location.host}} to get the current host (domain) of the page in Liquid. Within the context of an email, the email is scheduled asynchronously, so technically the Liquid context is not currently on a page in a browser. In this context, you can use {{platform_context.host}} to get the host of the "default domain". You can set the default domain of your instance in the domains tab in Siteglide Portal.