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
Recommended Solution:
The sender domain must add either an 'A' record or 'MX' record that returns the IP or IPs in question to their domain.
Workaround:
Disable DNS Validation for situations where RDNS exists, but 'A' or 'AAAA' does not match.
For both solutions: see Error: "554 5.7.1 Rejected message by MTA, RDNS record for connecting IP has no matching 'A' or 'MX' records"