As we know, a server installed as the mailbox role cannot be ran as a edge server.So we need some other 3-part components to bear these tasks. But what i want to sayis, if you are running everying thing(domain controller,dns server,exchange server)on a single server,there is really simple ways doing this.
1. Receiving external emails:
Creat MX record in your dns control panel, and then run the script:"Set-ReceiveConnector-Identity "Default <ServerName>" -PermissionGroups "AnonymousUsers"" inthe exchange shell.
2.Sending external emails:
Set a smtp connector in the exchange control panel and creat the policy(to send allthe out-going mail or depends on domain).
Go test your mailbox now.