SMTP Error Codes | Trend Micro Service Central

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When Deep Discovery Email Inspector blocks an email message based on Sender Filtering settings, Deep Discovery Email Inspector sends the following SMTP error codes to the upstream MTA.

Note:

Make sure that the upstream MTA can take the necessary pre-configured actions upon receiving these error codes. For example, creating an event log or sending notifications to senders.

Table 1.
Blocking Feature	SMTP Error Code	Message
Sender Filtering/Authentication settings (DHA Protection, Bounce Attack Protection, SMTP Traffic Throttling, SPF, DKIM, DMARC)	421	Block temporarily (Sender Filtering/Authentication)
	521	Block permanently (Sender Filtering/Authentication)
Email Reputation Service	450	Temporary denial of connection (450) for Zombie matches (ERS)
Email Reputation Service	550	Permanent denial of connection (550) for RBL+ matches (ERS)