How to Configure External URLs in Exchange Server 2019
Clients can connect to your new server from the Internet, you need to configure the external domains
After you've configured the external URL on the Mailbox server virtual directories, you need to configure your public DNS records for Autodiscover, Outlook Web App, and mail flow.
FQDN DNS record type Value
company.com MX mail.company.com
Mail.company.com A 192.168.1.20
Owa.company.com CNAME mail.company.com
Autodiscover.company.com CNAME mail.company.com