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 |
| 450 | 4.2.1 | G30 |
"Mailbox received too much email in a short period of time"
We rate limit inbound email to mailboxes to protect the entire system |
| 451 | 4.7.1 | G28 |
"Received too many messages from a new or untrusted IP"
We require that incoming IPs establish good sending behavior before sending large volumes of mail to our platform |
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 | 5.7.0 | 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 |
Mailfrom
| SMTP code | SMTP subcode | Support Code | Bounce Message / Meaning |
|---|---|---|---|
| 550 | 5.0.0 | G29 |
"Received too many messages from a sender mailbox"
We rate limit the email of senders to our platform to protect our system |
| 550 | 5.0.0 | G34 |
"Sender is not allowed to send with empty mail_from"
We require users to provide an email address in the MAIL FROM phase of the SMTP transaction |
Rcptto
| 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 |
| 554 | 5.7.1 | GE |
"5.7.1 {EMAIL}: Sender address rejected: Blocked by this recipient"
|
| 550 | 5.7.1 | G14 |
"5.7.1 {EMAIL}: Sender's MX record not allowed (G14)"
The senders domain's MX records match a blocklisted provider |
| 554 | 5.7.1 | G31 |
"5.7.1 ACL dns\_rbl; Client host [${ip}] blocked using ${zone}=${result} Senderscore. Please visit https://senderscore.org/rtbl/ for more information on why this message could not be delivered (G31)"
Sender's ip is blocked by Senderscore |
| 554 | 5.7.1 | G32 |
"5.7.1 ACL dns\_rbl; Client host [${ip}] blocked using ${zone}=${result} Spamhaus XBL. Please visit http://www.spamhaus.org/lookup/ for more information on why this message could not be delivered (G32)"
Sender's ip is blocked by Spamhaus XBL |
Data
| SMTP code | SMTP subcode | Support Code | Bounce Message / Meaning |
|---|---|---|---|
| 550 | 5.1.0 | R2 |
"Sender is not allowed to send from {MAIL FROM DOMAIN}"
|
| 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 |