Difference between subdomain.domain.com and domain.com in config file

Here is a basic question which has always baffled me in the phplist config file.

I have a registerd domain name called registereddomain.com and a server host called host.resgistereddomain.com which is the email sending domain.

What are the settings in the config file so that the recipient sees registereddomain.com and not host.registereddomain.com? Which seems to be happening now.

Basically I have a lot of registered domains all riding on one host.regisistereddomain.com and get confused as to what goes where in the phplist config file.

And finally, with google changing it’s bulk sender requirements, and now requireing TLS connections, how is that configured the config file for sending emails? Or at a minimum how does it interact if at all with phplist?

Thanks.