Everything you need to know about SMTP Error Codes

Last Updated, 2020-05-20 smtp

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


SMTP (Simple mail transfer protocol) is used to send emails across the network with the help of MTA (Mail transfer agent). On Basis the response from the client’s server SMTP returns a particular error code. These response codes might be one of the five standard classes.

Status Codes

Status code is differentiated into five categories as follows :

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

Enhanced Status Codes Registry

Informative Status Code




Unable to connect to server


Connection refused

Success Status Code : whenever a positive delivery code is returned from DSN




System status message


A response to the HELP command


The server is ready


The mail communication channel is getting closed


Requested mail action okay completed


mail will be forwarded because server was not able to find user on local


Cannot verify the user, but it will try to deliver the message anyway

Redirection Status Code




Start adding mail input

Persistent transient failure Status Code


Timeout connection problem


Service is unavailable due to a connection problem


The recipient’s mailbox has exceeded its storage limit


Not enough space on the disk


Recipient’s incoming mail queue has been stopped


The recipient’s server is not responding


The connection was dropped during the transmission


The maximum hop count was exceeded for the message


Message timed out because of issues concerning the incoming server


Routing error


User’s mailbox is unavailable


Aborted – Local error in processing


Too many emails sent or too many recipients


An error of your mail server

Server Permanent Error Codes




Syntax error


Syntax error in parameters or arguments


Bad sequence of commands, or requires authentication


Command parameter is not implemented

510 / 511

Bad email address


Host server for the recipient’s domain name cannot be found in DNS


Address type is incorrect


Size of your mail exceeds the server limits


Authentication problem


The recipient address rejected your message


Non-existent email address


User not local or invalid address – relay denied


Exceeded storage allocation


Mailbox name invalid


Transaction has failed

Diving deep with extented code description

2xx detailed

This is Success status which indicates that there is a positive response from the server which might have more information when the response has it respected 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. Since the status code response represents that the email address you are trying to reach is valid.

221 \ Service closing transmission channel

If there is an issue with authentication this status code response is received from the server. it can be an incorrect password or username or you might be trying the connect to the wrong mail server. you can check 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 response indicates recipient's MX as 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. the new priroty will available after the status code besides the response.

220 2.5.0 Requested mail action okay completed

This is success status which indicates your email has been successfully delivered at recipient's end without any issue.

4xx detailed

This status code are temporary blockage for your emails delivery which should retry sending those emails automatically. This blockage might be due to unusual traffic patterns from sending server IP address, emails having characteristics of spam or recipients MX is too busy accepting emails.

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

This status code errors when user raises complaints for the emails which has been sent from our mail server.

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

This status code errors when user raises complaints for the emails which has been sent from our 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 high volume of mails has been sent from your IP. ie, bulk emailing. Don't send email if you are receiving this status code
check for email subscription list and make sure you are sending emails to user who as subsribed.

421 Message temporarily deferred - [numeric code]

This status code response indicates your emails are postponed due to low connectivity percentage to MTA, Bad tokens within the message or unusal 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 few mintues

451 Message temporarily deferred - [numeric code]

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

5xx detailed

Error/Status Code


5.0.0 Address does not exist

Email id invalid or does not exists

5.1.0 Other address status

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

5.1.1 Bad destination mailbox address

Specified from address or domain mailbox does not exists (anything after '@').

5.1.2 Bad destination system address

Address does not exists or 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 your spelling of email address and resend

5.1.5 Destination mailbox address invalid

Mail address may be invailid or may have spelling mistakes.

5.1.6 Mailbox has moved

Mailbox may be moved to 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.

5.2.0 Other or undefined mailbox status

This error is usually return for which it has class (ie. 1xx,2xx,3xx,4xx,5xx).

5.2.1 Mailbox disabled, not accepting messages

Some changes need to made mandatory for resending mail

5.2.2 Mailbox full

Try increasing your emailbox space capacity

5.2.3 Message length exceeds administrative limit.

The Size of your email has exceeded its limit try removing unwanted text.

5.2.4 Mailing list expansion problem

mailbox saves your email address to whom you are sending or receiving emails, there is problem saving that email addresses.

5.3.0 Other or undefined mail system status

Mail has been accepted normally

5.3.1 Mail system full

mail system at client's is not able accept emails

5.3.2 System not accepting network messages

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

5.3.3 System not capable of selected features

5.3.4 Message too big for system

Message character limit has exceeded

5.4.0 Other or undefined network or routing status

Mail accepted routing status is not available

5.4.1 No answer from host

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

5.4.2 Bad connection

unable to connected or delay in connection

5.4.3 Routing server failure

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

5.4.4 Unable to route

When emailid or hosts provided is invalid.

5.4.5 Network congestion

Multiple connection on single channel.

5.4.6 Routing loop detected

Sending message to same PMTA from you have sent.

5.4.7 Delivery time expired

Mail is queued for 2-3 days after that it is removed by returning above messagae

5.5.0 Other or undefined protocol status

Something went wrong with protocol while delivering mail which can be well expressed with other status code.

5.5.1 Invalid command

out of sequence or unsupported command used while sending mail

5.5.2 Syntax error

unable to interprete the command due wrong syntax or it is unrecognized.

5.5.3 Too many recipients

Number of recipient is greater than the amount of recipient deliver by protocol

5.5.4 Invalid command arguments

the arguments were out of range or represented unrecognized features

5.5.5 Wrong protocol version

protocol version is mis-match which can't be solved at one go.

5.6.0 Other or undefined media error

Issue with content of the message which cannot be delivered.

5.6.1 Media not supported

Media is not supported by protocol or the system who is going to formard the mail.

5.6.2 Conversion required and prohibited

Message should be converted before it is send/delivery else it will be not delivered.

5.6.3 Conversion required but not supported

Message must be converted in order to be forwared but partically it is not possible.

5.6.4 Conversion with loss performed

This is Warning to the sender that While conversion data may be lost.

5.6.5 Conversion failed

Message conversion was unsuccessful

5.7.0 Other or undefined security status

Something went wrong while validating authentication which can be expressed with other status code.

5.7.1 Delivery not authorized, message refused

Authentication was not successful due to which email was not sent.

5.7.2 Mailing list expansion prohibited

Sending mail is not allowed to particular recipient.

5.7.3 Security conversion required but not possible

Conversion was required to send mail securely which is not possible.

5.7.4 Security features not supported

Authentication not supported at delivery end.

5.7.5 Cryptographic failure

Unable to decrypt the message because the key was wrong or invalid.

5.7.6 Cryptographic algorithm not supported

Unable to decrypt the message because alogrithm was not supported.

5.7.7 Message integrity failure

The message is corrupted or altered

Stages where error codes are possible

Below is an simple example for sending email using telnet and you can check the response code after every command.

[[email protected]_wifi ~]$ telnet 587
Connected to
Escape character is '^]'.
#-----------EHLO or HELO---------------
250-SIZE 50000000
250 DSN
mail from: [email protected]
250 2.1.0 Ok [success output]
# rcpt to: [email protected]
250 2.1.5 Ok [success output]
# 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
250 2.0.0 Ok: queued as E2R572042D75 [success output]
quit # quit from the telnet command prompt
221 2.0.0 Bye
Connection closed by foreign host.

I have segregated the above example in below structure where you can easily understand the status code.

      S: 220
      E: 554
      S: 250
      E: 504, 550
      S: 250
      E: 552, 451, 452, 550, 553, 503
      S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
      E: 550, 551, 552, 553, 450, 451, 452, 503, 550
      I: 354 -> data -> S: 250
                        E: 552, 554, 451, 452
      E: 451, 554, 503
      S: 250
      S: 250, 251, 252
      E: 550, 551, 553, 502, 504
      S: 250, 252
      E: 550, 500, 502, 504
      S: 211, 214
      E: 502, 504
      S: 250
      S: 221
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