Custom Domain Email

[KalenaJames]

trafficstars

Subscribers to my CRM sign up with a name, a custom domain email, and a password. When the user signs up or tries to reset the password using their customized domain email, they do not receive an email from our firebaseapp.com email account. Why is this? And without having every client check their DNS records, what code or configuration is necessary to ensure our emails are received?