Legitimate emails are being stopped by Symantec Messaging Gateway (SMG) due to the following conditions:
Message Audit Log entries will show:
Rejected message by MTA, RDNS record fro connecting IP has no matching 'A' or 'MX' records
The sender domain must add either an 'A' record or 'MX' record that returns the IP or IPs in question to their domain.
Disable DNS Validation for situations where RDNS exists, but 'A' or 'AAAA' does not match.