Interpreting Bounce Codes

[Kiersti_Esparz1]

*Updated in September 2024

When you add the Email Invalid Cause to display as a Column to any of your Deliverability Smart Lists, you will see a code value*, and potentially a suffix as well, to help you understand the reason for a Hard Bounce.  Soft Bounce reasons can't be viewed through a Smart List report, only one at a time in a lead's activity log.

• Codes in the 400 range are generally Soft Bounces
• Codes in the 500 range are generally Hard Bounces

The SMTP response codes below are common codes that a mail administrator may use.  Mail administrators and providers can also create unique SMTP response codes and details.  When reviewing bounces the additional information included within the bounce details will help to diagnose why mail was rejected, so adjustments can be made to avoid the same bounce type in the future.

Traditional Bounce Codes

Code	Meaning
211	System status, or system help reply
214	Help message
220	example.com service ready
221	example.com service closing transmission channel
250	Requested mail action okay, completed
251	User not local; will forward to forward-path
354	Start mail input; end with .
421	example.com service not available, closing transmission channel
450	Requested mail action not taken: mailbox unavailable
451	Requested action aborted: error in processing
452	Requested action not taken: insufficient system storage
500	Syntax error, command unrecognized
501	Syntax error in parameters or arguments
502	Command not implemented
503	Bad sequence of commands
504	Command parameter not implemented
550	Requested action not taken: mailbox unavailable
551	User not local; please try
552	Requested mail action aborted: exceeded storage allocation
553	Requested action not taken: mailbox name not allowed
554	Transaction failed

Enhanced Bounce Codes

If a suffix appears after one of the codes above, it is an enhanced Bounce code

*Mail server administrators can create custom messages that accompany bounce codes

Code	Enhanced Code	Message	Comment
	4.3.0	Resource limitations, please try later	See message.
	4.3.5	configuration error, please try later	See message.
	4.3.5	message not yet parsed, can't reparse	See message.
	4.3.5	message parse and assemble didn't round-trip	See message.
	4.7.5	[internal] SSL certificate subject does not match host	See message.
	4.7.5	[internal] remote node SSL certificate not signed by a valid CA	See message.
	4.7.5	[internal] SSL cert must be signed by a valid CA	See message.
	4.7.5	[internal] SSL cert common name does not match host	See message.
	5.5.0	error while reading body	See message.
	5.7.0	[internal] Message manually purged	Used console command, "fail domain name"
	5.7.0	[internal] rejected by policy	See message.
421		This message is being 421'd	Service is unavailable
421	4.3.0	message is too large to process at this time	See message.
421	4.3.4	allocated resources exceeded	System outage
421	4.4.0	[internal] no MXs for this domain could be reached at this time	See message.
421	4.4.5	Service unavailable, concurrency limit reached	System outage
421	4.4.5	Service unavailable	System outage
450	4.4.1	[internal] Connection Timed Out	See message.
451		[internal] memory shortage while determining tls parameters	See message.
451	4.3.0	transaction failed	See message.
451	4.4.1	[internal] No valid hosts (unable to make any connections)	See message.
451	4.4.1	[internal] No valid hosts	See message.
451	4.4.2	[internal] send key failed	Cluster-related error
451	4.4.2	[internal] send metadata failed	Cluster-related error
451	4.4.2	[internal] no METADATA response	Cluster-related error
451	4.4.2	[internal] no banner	See message.
451	4.4.2	[internal] send HELO/EHLO failed	See message.
451	4.4.2	[internal] no HELO/EHLO response	See message.
451	4.4.2	[internal] XCLIENT argument hook failed	See message.
451	4.4.2	[internal] send XCLIENT failed	See message.
451	4.4.2	[internal] no XCLIENT response	See message.
451	4.4.2	[internal] MAIL FROM argument hook failed	See message.
451	4.4.2	[internal] MAIL FROM arguments too long	See message.
451	4.4.2	[internal] send MAIL FROM failed	See message.
451	4.4.2	[internal] no MAIL FROM response	See message.
451	4.4.2	[internal] RCPT TO argument hook failed	See message.
451	4.4.2	[internal] RCPT TO arguments too long	See message.
451	4.4.2	[internal] send RCPT TO failed	See message.
451	4.4.2	[internal] no RCPT TO response	See message.
451	4.4.2	[internal] send DATA failed	See message.
451	4.4.2	[internal] no DATA response	See message.
451	4.4.2	[internal] send BODY failed	See message.
451	4.4.2	[internal] no BODY response	See message.
451	4.4.2	[internal] send RSET failed	See message.
451	4.4.2	[internal] no RSET response	See message.
451	4.4.2	[internal] connection closed by remote host	See message.
451	4.4.2	[internal] send metadatadicts failed	Cluster-related error
451	4.7.5	[internal] send XCLUSTERMETADATA failed	Cluster-related error
451	4.7.5	[internal] no clustermetadata go-ahead	Cluster-related error
451	4.7.5	[internal] send STARTTLS failed	See message.
451	4.7.5	[internal] no tls go-ahead	See message.
451	4.7.5	[internal] TLS negotiation failed	See message.
451	4.7.6	[internal] XCLUSTERMETADATA required but not advertised	Cluster-related error
451	4.7.6	[internal] STARTTLS required but not advertised	See message.
451	4.7.6	[internal] XCLIENT required but not advertised	See message.
451	4.7.6	[internal] XCLUSTERMETADATA supported version not advertised	Cluster-related error
452	4.5.3	Service unavailable, batch limit reached	See message.
452	4.5.3	Service unavailable, recipients per connection limit reached	See message.
452	4.5.3	Service unavailable, recipients per message limit reached	See message.
454		certificate not yet valid: possible clock skew	SSL related
454		certificate expired	SSL related
454	4.4.4	[internal] no MX or A for domain	See message.
500	5.5.2	unrecognized command	See message.
501	5.5.1	MAIL FROM already established	See message.
501	5.5.2	RCPT TO syntax error	Triggered by an invalid address
501	5.5.2	MAIL FROM syntax error	Triggered by an invalid address
501	5.5.2	invalid EHLO	See message.
501	5.5.2	invalid HELO	See message.
501	5.5.2	MAIL FROM syntax error	See message.
501	5.5.2	RCPT TO syntax error	See message.
501	5.5.4	invalid arguments to RSET	See message.
501	5.5.4	unexpected argument to DATA	See message.
501	5.5.4	Syntax error (no parameters allowed)	See message.
501	5.5.4	unexpected command	See message.
503		RCPT TO without MAIL FROM	See message.
503	5.5.1	DATA without MAIL FROM	See message.
503	5.5.1	DATA without RCPT TO	See message.
503	5.5.1	bad sequence of commands	See message.
550	5.3.0	[internal] message body missing	See message.
550	5.4.4	[internal] NULL MX domain does not accept mail	See message.
550	5.5.1	RFC2821 HELO/EHLO needed	See message.
550	5.6.0	Lone CR or LF in headers (see RFC2822 section 2.2)	See message.
550	5.6.0	Missing CR LF . CR LF	See message.
550	5.6.0	Required headers not found (see RFC2822 section 3.6)	See message.
550	5.6.0	Header not terminated by CR LF (see RFC2822 section 2.2)	See message.
550	5.6.0	Invalid header found (see RFC2822 section 3.6)	See message.
550	5.6.0	Missing required RFC2822 Date field (see RFC2822 section 3.6)	See message.
550	5.6.0	Lone CR or LF in body (see RFC2822 section 2.3)	See message.
550	5.6.0	Header line too long (see RFC2822 section 3.5)	See message.
550	5.6.0	Body line too long (see RFC2822 section 3.5)	See message.
550	5.7.1	relaying denied	See message.
550	5.7.1	[internal] discarded by policy	See message.
550	5.7.1	[internal] processed as FBL message, and disposition is blackhole	FBL module
550	5.7.1	550 [internal] [oob]	bounce_logger module
551	5.4.6	[internal] Private/Loopback Address	See message.
551	5.7.1	recipient blackholed	See message.
552	5.3.4	message size limit exceeded	See message.
554		No Valid Domain	See message.
554	5.4.4	[internal] Domain Lookup Failed	See message.
554	5.4.5	[internal] Delivery not attempted (message expired)	See message.
554	5.4.7	[internal] exceeded max retries without delivery	See message.
554	5.4.7	[internal] exceeded max time without delivery	See message.
554	5.7.0	[internal] Blackholed	"Blackhole" option is set to "internal"
554	5.7.0	[internal] Message manually purged	Purged messages from the system console