If left completely unconfigured, the mail system on the PABX will attempt to use standard Internet mail rules for delivery, so configuring MX records in DNS may be sufficient.
A mail “SmartHost” may alternatively be configured into global settings to direct the PABX to send all mail via this single host.
Some (many) mail servers will not accept email from servers where the hostname and/or IP address cannot be correctly resolved in DNS, so it may be necessary to add the PABX to your internal DNS server.