The author voluntarily contributed this tutorial as a part of Pepipost Write to Contribute program.

Introduction

SMTP (Simple Mail Transfer Protocol) is used to send emails across the network with the help of a MTA (Mail Transfer Agent). When emails are sent, the response from the client’s SMTP server may return a particular error code. Such response codes might be from one of the five standard classes.

Status Codes

SMTP response codes are updates sent by an SMTP server during the mail delivery process. They are sent in response to commands issued by an SMTP client using a 3-digit format. These numerical codes indicate a status or error pertaining to a message transmission, allowing senders to troubleshoot sending issues. 

Status code is differentiated into five categories as follows:

• 1xx returns informational code i.e., your request is received successfully and sent for further processing.
• 2xx returns a success code i.e., your request is understood and accepted.
• 3xx returns a redirection code i.e., your request needs few actions to be taken to be completely accepted.
• 4xx returns a client error code i.e., your request is incorrect, might be a syntax issue or contains an issue that can't be fulfilled.
• 5xx returns a server error code i.e., your request was valid but the server failed to process due to internal error.

Here’s what the second digit identifies: 
The third digit provides more detail and depends on the class and the subject.

1. Informative status code
   
2. Success status code  
   
   
3. Redirection status code
   
4. Persistent transient failure status code

Server permanent error codes

Diving deep with extended code description

2xx detailed

This is a success status, which indicates that there is a positive response from the server. This might have more information when the response has its respective subcodes (2XX X.X.X) and response.

250 2.1.5 Destination address valid or 250 2.1.5 Recipient OK

This should be treated as a positive remark for delivery. The status code response represents that the email address you are trying to reach is valid.

221 \ Service closing transmission channel

This status code response is received from the server if there is an issue with authentication. It can be an incorrect username or password or you might be trying to connect to the wrong mail server. You can check if your SMTP access is enabled or not for sending email from your email id or try changing the authentication method.
If you are confident with your username and password and still getting the error, try using SSL/TLS.

250 2.3.0 Other or undefined mail system status

This can be considered as a positive response because this status code indicates recipient's MX has successfully received your email but has reverted with null response.

250 2.3.6 Requested priority was changed

The email has been accepted for delivery but the delivery priority was changed.

220 2.5.0 Requested mail action okay completed

This is a success status, which indicates your email has been successfully delivered at recipient's end.

4xx detailed

This status code is a temporary blockage for your email delivery. It retries sending those emails automatically. Such blockages might be due to unusual traffic patterns from the sending server IP addresses, emails having characteristics of spam, or recipient’s MX being busy accepting emails.

.

421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.55.1

This occurs when a user raises complaints for the emails which have been sent from your mail server.

421 4.7.0 [TS02] Messages from x.x.x.x ... deferred due to excessive user complaints - 4.16.56.1

This occurs when a user raises complaints for the emails which have been sent from your mail servers on a shared IP.

421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred

This status code indicates that a high volume of mails has been sent from your IP. i.e., bulk emailing. Stop sending email if you are receiving this status code. Check your email subscription list and make sure you are sending emails to only users who have subscribed.

421 Message temporarily deferred - [numeric code]

This status code response indicates your emails are postponed due to low connectivity to MTA, bad tokens within the message, or unusual traffic of emails having PDF files in it.

421 Resources temporarily unavailable. Please try again later

The recipient's MX is too busy. Try to resend those emails after a few minutes.

451 Message temporarily deferred - [numeric code]

This status code is due to excessive traffic or IP data query failure.

5xx detailed

Error/Status Code

Description

5.0.0 Address does not exist

Email id is invalid, or it does not exist

5.1.0 Other address status

A problem occurred during the delivery of the message to an email address.

5.1.1 Bad destination mailbox address

The specified ‘from’ address or domain mailbox does not exist (anything after '@').

5.1.2 Bad destination system address

Address does not exist or it is not able to accept mails

5.1.3 Bad destination mailbox address syntax

Invalid or corrupted character in email address

5.1.4 Destination mailbox address ambiguous

Check the spelling of your email address and resend

5.1.5 Destination mailbox address invalid

Mail address may be invalid or it may have spelling mistakes.

5.1.6 Mailbox has moved

Mailbox may be moved to a new location

5.1.7 Bad sender’s mailbox address syntax

Sender's address is invalid syntactically

5.1.8 Bad sender’s system address

Sender's system in the address does not exist or is incapable of accepting messages.

5.2.1 Mailbox disabled, not accepting messages

The mailbox exists, but is not accepting messages

5.2.2 Mailbox full

Try increasing your mailbox space capacity

5.2.3 Attachment is large

The message could not be delivered because it had larger attachment than the recipient's email system allows

5.2.4 Mailing list expansion problem

There is a problem saving the email address.

5.3.0 Other or undefined mail system status

The destination system exists and normally accepts mail, but something about the system has caused the bounce back

5.3.1 Mail system full

The clients’ mail system is not able to accept emails

5.3.2 System not accepting network messages

You need to whitelist the email address through your administrator as it has been blocked.

5.3.3 System not capable of selected features

Selected features specified for the message are not supported by the destination system

5.3.4 Message too big for system

Message character limit has exceeded

5.4.0 Other or undefined network or routing status

Contact the sender's email or network support team as there was a network issue, possibly with the sender's email server

5.4.1 No answer from the host

Configuration issue at recipient's server. You can send again once the issue is resolved at the recipient's end

5.4.2 Bad connection

Unable to connect or delay in connection

5.4.3 Routing server failure

Error is generated by the sender's server and you need to contact your ISP/ESP.

5.4.4 Unable to route

When email id or hosts provided is invalid.

5.4.5 Network congestion

Multiple connections on a single channel.

5.4.6 Routing loop detected

Send a message to the same PMTA you have sent.

5.4.7 Delivery time expired

Mail is queued for 2-3 days and thereafter it’s removed by returning the message “ Delivery time expired”

5.5.0 Other or undefined protocol status

The message could not be delivered because the mailbox is unavailable

5.5.1 Invalid command

Out of sequence or unsupported command used while sending mail

5.5.2 Syntax error

Unable to interpret the command due to wrong syntax or it is unrecognized.

5.5.3 Too many recipients

Number of recipients is greater than the set limit.

5.5.4 Invalid command arguments

The arguments were out of range or represented unrecognized features

5.5.5 Wrong protocol version

Sending and receiving email systems are incompatible

5.6.1 Media not supported

Media is not supported by protocol or the system that is forwarding the mail.

5.6.2 Conversion required and prohibited

Message should be converted before it is sent or else it will not be delivered.

5.6.3 Conversion required but not supported

The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path.  (This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop)

Stages where error codes are possible

Below is a simple example for sending email using telnet. You can check the response code after every command.

[vikram.sahu@Nl587_wifi ~]$ telnet smtp.pepipost.com 587

Trying 138.197.56.58...

#-----------CONNECTION ESTABLISHMENT---------

Connected to smtp.pepipost.com.

Escape character is '^]'.

220 ESMTP SMTPNY-LB2 Ready

#-----------EHLO or HELO---------------

EHLO pepipost.com

250-smtpbny-lb2.pepipost.com

250-PIPELINING

250-SIZE 50000000

250-VRFY

250-ETRN

250-STARTTLS

250-AUTH PLAIN LOGIN

250-AUTH=PLAIN LOGIN

250-ENHANCEDSTATUSCODES

250-8BITMIME

250 DSN

#------------MAIL-----------------

mail from: [email protected]

250 2.1.0 Ok [success output]

#------------RCPT-----------------

# rcpt to: [email protected]

250 2.1.5 Ok [success output]

#------------DATA-----------------

# data # type this to pass input of email body

354 End data with <CR><LF>.<CR><LF> [success output ]

This is test mail from your local host server #mail body

. # dot(.) for completing the mail body

#------------NOOP-----------------

250 2.0.0 Ok: queued as E2R572042D75 [success output]

quit # quit from the telnet command prompt

#-----------QUIT------------------

221 2.0.0 Bye

Connection closed by foreign host.

The above example is further detailed below to easily understand the status code.

  CONNECTION ESTABLISHMENT

      S: 220

      E: 554

   EHLO or HELO

      S: 250

      E: 504, 550

   MAIL

      S: 250

      E: 552, 451, 452, 550, 553, 503

   RCPT

      S: 250, 251 (but see section 3.4 for discussion of 251 and 551)

      E: 550, 551, 552, 553, 450, 451, 452, 503, 550

   DATA

      I: 354 -> data -> S: 250

                        E: 552, 554, 451, 452

      E: 451, 554, 503

   RSET

      S: 250

   VRFY

      S: 250, 251, 252

      E: 550, 551, 553, 502, 504

   EXPN

      S: 250, 252

      E: 550, 500, 502, 504

   HELP

      S: 211, 214

      E: 502, 504

   NOOP

      S: 250

   QUIT

      S: 221

Hope this tutorial helped you!

We are a community of email enthusiasts. If you have further queries, drop an email to [email protected] and we would be happy to help you.

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