SMTP Response
SMTP Responses provide a brief explanation into the reason behind why your mail was not delivered. All response codes / subcodes must comply with relevant RFC specifications among different mail providers making it easier to fix issues. Error codes fall into two main categories : Deferred (4xx) and Rejected (5xx) these codes show us what is causing your temporary or permanent issues and enable you to fix these errors or issues.
Deferrals (4xx Codes)
A 4xx error indicates that there is a temporary failure, this means mail cannot be sent at the current time, the best thing to do with these issues is to wait a while and try to resend your email later. An example of what can cause these temporary failures would be if your mailbox was receiving too much mail in a short period of time, this means the servers need to catch up with processing the mail.
Rejections (5xx Codes)
A 5xx error indicates a permanent failure. This error indicates an error in your emails content, formatting or recipients that prevents delivery. These issues must be addressed and edited before re-attempting delivery. The error contained in the SMTP response will indicate the specific problem with the message. For example if you are trying to send an email to an inbox that does not exist.
Deferral Codes (4xx Codes)
Connect
| SMTP Code | SMTP SubCode | Support Code | Bounce Message |
| 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 |
| 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 |
| 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 (5xx Codes)
| SMTP Code | SMTP SubCode | Support Code | Bounce Message |
| 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 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 |
RCPTTP
| SMTP Code | SMTP SubCode | Support Code | Bounce Message |
| 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 |
| 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 |
