You are getting messages rejected by Symantec Messaging Gateway (SMG) because the domain of the e-mail address in the From: field does not have valid DNS records.
553 5.1.8 Rejected message by MTA, MAIL-FROM domain does not exist in DNS
The sender's DNS records do not have a proper 'A', 'AAAA', or 'MX' record that matches the domain in the From: field of the e-mail header.
You have "Reject messages where the domain provided in the MAIL FROM address has neither an 'A', nor an 'AAAA', nor an 'MX' record in DNS" enabled.
The best solution is to have the owner of the IP addresses' DNS records add a 'A', 'AAAA', or 'MX' record to resolve that IP to a fully qualified domain name (FQDN).
You can also disable the check in SMG: