I guess if the main server IP/Hostname has rDNS (IPv4 & IPv6) with valid DKIM/SPF records, then I guess that'll work - I'm going by if most of us use the xxx.company-name.com for Hostnames.
But an external smtp will help - some of us use Google Apps with configured DKIM & SPF, so if we were able to use their smtp, we would 99% know the emails wouldn't be marked as potential spam.