System Information
Response Codes

SMTP Responses

SMTP responses provide a brief explanation of why an email message was not delivered. Response codes, subcodes, and messages vary among mail providers, but must comply with relevant RFC specifications. Response codes for failed delivery fall into two categories: Deferred (4xx) and Rejected (5xx). Deferral and Rejection codes indicate temporary and permanent issues respectively that are preventing delivery to one or more recipients.

DEFERRALS (4XX CODES)

A 4xx error indicates a temporary failure, which means that delivery cannot be completed at this time. In the event of a deferral the sender should retry delivery later. An example of a temporary failure would be a mailbox receiving too much mail in a short period of time, causing some mail to be deferred for later delivery.

REJECTIONS (5XX CODES)

A 5xx error indicates a permanent failure. This error means there is a problem with the message content, formatting, or recipients that prevents delivery. These issues must be addressed before delivery can be re-attempted by resending the message. In most cases, the message contained in the SMTP response will indicate the specific problem with the message. An example of a permanent failure is a message sent to a recipient that does not exist.

Deferral Codes


Connect

SMTP code SMTP subcode Support Code Bounce Message / Meaning
421 G2 "Too many concurrent connections from this client (G2)"
Too many concurrent connections from client IP
450 4.7.1 G1F "4.7.1 Client host {IP} has no PTR record (G1F)"
We require that incoming IPs have a valid PTR

Rcptto

SMTP code SMTP subcode Support Code Bounce Message / Meaning
450 4.4.1 G6 "4.4.1 {EMAIL}: The recipient's server was temporarily unavailable to accept your message (G6)"
A temporary failure while attempting to verify the recipient address

Data

SMTP code SMTP subcode Support Code Bounce Message / Meaning
450 4.4.6 G16 "4.4.6 Routing loop detected (G16)"
An undeliverable mail loop was detected
450 4.4.0 G1E "4.4.0 {EMAIL}: Temporary DMARC DNS lookup failure (G1E)"
A temporary failure while looking up sender's DMARC policy

Rejection Codes


Connect

SMTP code SMTP subcode Support Code Bounce Message / Meaning
550 5.1.1 G8, G9 "5.1.1 {EMAIL}: Email address could not be found, or was misspelled (G8, G9)"
Recipient address does not exist on our system
550 5.7.1 G3, G4, G5 "5.7.1 Virus found (G3,G4,G5)"
Message was identified as a virus by our content filter
550 5.7.1 G15 "5.7.1 Email rejected per DMARC policy for {DMARC DOMAIN} (G15)"
Message failed DMARC validation
550 5.7.0 G18 "5.7.0 header_contenttype_1 (G18)"
Rejected for matching a known virus signature
550 5.7.0 G19 "5.7.0 header_contenttype_2 - Multi-part MIME messages are rejected as dangerous (G19)"
Partial MIME messages are not supported
550 5.7.0 G1A "5.7.0 mime_banned_file - Your message has been rejected because it contains a banned file attachment (G1A)"
Executables in compressed attachments are not allowed
550 5.7.0 G1B "5.7.0 mime_banned_file - Your message has been rejected because it contains a banned file attachment (G1B)"
File TypesFile Types that exploit vulnerabilities in Windows programs are not allowed
550 5.7.0 G1C "5.7.0 mime_banned_file - File attachment is not allowed because they can be used to exploit Winzip (G1C)"
Messages with attachment file extensions linked to WinZip vulnerabilities are not allowed
550 5.7.0 G1D "5.7.0 mime_banned_file - CLSID file extensions are dangerous (G1D)"
Messages with attachment file names containing Windows classIDs are not allowed

Rcptto

SMTP code SMTP subcode Support Code Bounce Message / Meaning
550 5.7.0 G1A "5.7.0 mime_banned_file - Your message has been rejected because it contains a banned file attachment (G1A)"
Block messages with .exe files in attached archives

Data

SMTP code SMTP subcode Support Code Bounce Message / Meaning
550 5.1.1 G8, G9 "5.1.1 {EMAIL}: Email address could not be found, or was misspelled (G8, G9)"
Recipient address does not exist on our system
550 5.7.1 G7 "5.7.1 {EMAIL}: The recipient's server refused to accept your message (G7)"
A permanent failure while attempting to verify the recipient address
550 5.7.1 GE "5.7.1 {EMAIL}: Recipient not allowed (GE)"
Recipient is blacklisted
550 5.7.1 GC, G12 "5.7.1 {EMAIL}: Sender not allowed (GC, G12)"
Rejected for sender being globally blacklisted
550 5.7.1 G14 "5.7.1 {EMAIL}: Sender's MX record not allowed (G14)"
The senders domain's MX records match a blacklisted provider