They're using Gmail's SMTP Service, there's some guides/info out there, although you always fail SPF with it which can end up with your emails going to spam.
You could find a proper email sending service like ses/smtp go and I assume outlook would let you send out through it like gmail would. If you want a more proper custom email solution there's better options like Fastmail/protonmail, Google Workspace/Microsoft 365, free options people have talked about before like Zoho Mail (which is free for one custom domain).
I wouldn't trust Email Routing with any mail that is important to receive unless you have full control over the destination to stop it from spamming/dropping forwarded mail, forwarding mail is bleh.
