how does the appointment+ send confirmation emails?

I am testing appointments+. Yahoo and Gmail does not receive any confirmation emails. I have tried many settings, pay/no pay, and WP-Mail-SMTP plugin. no confirmation email is sent outside my server. I can receive emails on my host's server. So the question is does appointment+ use its own server or my host server?
I believe Yahoo and Gmail do not receive emails because it looks suspicious (i.e. email in from field not same as server).