You are not getting mail messages to complete in Symantec Messaging Gateway (SMG) due to unavailability of reverse DNS record for an IP address.
554 5.7.1 Rejected message by MTA, EHLO domain does not exist in DNS
The sender's DNS records do not have a valid 'A', 'AAAA', nor 'MX' entry for the domain presented in the HELO/EHLO commands.
You have rejection of messages that fail HELO/EHLO reverse DNS checks enabled.
The best solution is to have the owner of the IP addresses' DNS records add a PTR record to resolve that IP to a fully qualified domain name (FQDN).
You can also disable the check in SMG: