I'm hosting a forum at subdomain1.example.com. For activation emails etc, I have a Mailgun server at mg.subdomain1.example.com. The Mailgun server can only send emails, no receives, so I bought an email plan from GoDaddy and installed this at domain.com. Then because I like it, I added MX records, so I can access the emails through Gsuite.
But once I started with the Gsuite setup, I wondered if I needed that GoDaddy email plan at all? Could I have set it up without it?
Every domain or subdomain can have email services, which are then controlled through domain name records called MX records. You will no doubt see these in your domain control panel. Email from the internet is sent to the servers specified in those MX records and only those.
So if your domain only has MX records for Google GSuite, you can't be using something else (inbound).
Outbound is slightly different, in that services like Mandrill/Mailgun etc can send on your behalf.. but thats another topic.
So the answer to your question depends on the settings in the domain name for each domain or subdomain that you are referring to.
MXToolbox is quite a helpful site to quickly check settings without logging into a control panel.
