If emails from external senders are bouncing and encountering a 554 error in Trend Micro Email Security (TMES), it is likely that the original sender is not from inside the organization. To resolve this issue, a rule needs to be added in the existing TMES Outbound Rule in the Exchange.
The rule should be set to allow the forwarding of email which is not from the organization. Specifically, the rule should state that the sender is external/internal and is located 'InOrganization'.