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Summary of common email return codes and solutions

Last Updated: Dec 15, 2017

Codes of successful email delivery

If an email is sent successfully, DirectMail or the recipient server will return a code: “250 send mail ok”, which indicates that the email request operation is completed, and the email has been sent to the recipient server successfully.

If an email is sent successfully, but the recipient does not receive it. The solution is as follows:

  • Ask the recipient to check whether the email was put into the spam email folder.
  • Ask the recipient to add your sender address to the whitelist, and then send the email again.
  • If the email still cannot be received after adding the sender address to the whitelist, ask the recipient to submit this issue to the email service provider for resolution.

Codes of failed email delivery

If an email fails to deliver, the related error codes will be returned. The 4xx return codes are indicative of temporary errors. The 5xx return codes are indicative of permanent errors. If a temporary error is returned by the recipient server (the return code is 4xx), the email will be resent a few times.

Return codes can differ for each email service providers. They represent different reasons why emails fail to deliver.

Common error codes from DirectMail

Error codes from DirectMail refer to the error codes returned during a mailing request to DirectMail, or the return codes returned by DirectMail during delivery.

Type of return code Return code Solution
Authentication failed 535 Authentication failure Check whether the email sending address or password is correct.
Number of recipient addresses exceeded 556 The number of RCPTs is over the size limit The recipient address limit has been exceeded.
The limit for one email is 30. Reduce the number of recipients.
Email size exceeded 557 The length of DATA content has reached the maximum threshold The email size limit is exceeded. Reduce the size of the email.
Invalid recipient address 559 Invalid rcptto The recipient address is invalid.
Check whether the address is correct. If it is correct, open a ticket for solution.
MX resolution query failed 423 Dns resolve error
524 Host not found by dns resolve
526 No data by dns resolve
The recipient domain name MX resolution query failed.
Ask the recipient to check the resolution (MX) record of the domain name.
Connection failed 427 Socks Connect to UNREACHABLE host The target host cannot be reached. This is typically because the resolution (MX) record of the recipient domain name does not exist.
Ask the recipient to check the resolution (MX) record of the domain name.
Email sending quota exceeded 552 Monthly sending quota exceeded
552 Daily sending quota exceeded
552 Not enough sending quota
Email sending quota has been exceeded.
Spam rejection error 554 Reject by content spam The email has been tagged as a spam email. Modify the content and send again.
Also, open a ticket providing the email sender address, recipient address, and send time to DirectMail customer service to assist them in finding out whether or not it is a false positive brought about by too strict standards.
Abnormal email sender account status 551 Main account unavailable The current account is restricted or in an abnormal status.
Log on to the DirectMail console to view the current account status.

Common error codes from the recipient servers

Error codes from the recipient servers refer to the return codes received from the recipient inbox during delivery.

Note: xx represents any symbol; $EMAIL represents email address; $IP represents email sender IP; $DOMAIN represents domain name.

Type of return code Return code Solution
Recipient address not found 521 Recipient address rejected
550 5.1.1 https://support.google.com/mail/?p=NoSuchUser
550 5.1.1 https://support.google.com/mail/answer/6596
550 Invalid User
550 Mailbox not found
550 user not exist
550 user not exist
550 User not found
554 delivery error: dd Sorry your message to $EMAIL cannot be delivered. This account has been disabled or discontinued
554 delivery error: dd This user doesn’t have a $DOMAIN account ($EMAIL)
The email return code includes the following information:
account does not exist
invalid address
invalid mailbox
invalid recipient
invalid user
is not a valid mailbox
is not a valid user
mailbox not found
no mailbox by that name
no mailbox here by that name
no such address
no such mailbox
no such recipient
no such user
recipient does not exist
recipient is not exist
recipient not found
recipient unknown
unknown or illegal alias
unknown user
user not exist
user not found
user unknown
The recipient address is not found or is disabled.
Confirm the address with the recipient.
Account blacklist 550 Mail is rejected by recipients
550 sender in prision
550 User in blacklist
The recipient or the system has blacklisted the sender account.
Ask the recipient to check their blacklist and filter policy.
If the account is not blacklisted, ask the recipient to submit this issue to their email service provider for resolution.
IP blacklist 450 DT:RBL
554 HL:IPB
550 DY-001
550 DY-002
550 OU-001
550 OU-002
550 SC-003
550 SC-004
550 in an RBL
550 ip in prision
550 Your IP address is blacklisted
550 5.7.1 Service unavailable; client [$IP] blocked using $DOMAIN
553 5.7.1 [BLxx] Connections will not be accepted from $IP
554 5.7.1 Client host [$IP] blocked using $DOMAIN
550 5.7.1 Mail from IP $IP was rejected due to listing in Spamhaus SBL
550 IP $IP is blocked
554 IP in blacklist
598 ip address in blacklist
The email return code includes the following information:
denied by RBL
in a DNS blacklist
in the RBL
is filtered by RBL
is listed in RBL
The email sender IP is blacklisted.
Submit a ticket for solution.
Recipient address confined 450 4.2.1 https://support.google.com/mail/answer/6592
550 Mailbox unavailable or access limited
The recipient has received a large number of emails. To avoid malicious attacks, the recipient address is temporarily confined.
Wait a moment, and then resend the email to the recipient.
IP frequency limit exceeded 421 HL:ICC
421 HL:IFC
421 RP-001
421 RP-002
421 RP-003
550 Connection frequency limited
550 IP frequency limited
The email sender IP frequency limit has been exceeded.
Stop sending emails temporarily, and then retry sending the email.
If the IP frequency exceeds the daily email sending quota, no email is allowed to be sent on that day.
The frequency limit is confidential data belonging to the recipient and is not publicly available.
Email sending account frequency limit exceeded 451 MI:SFQ
550 MI:STC
550 RP:TRC
554 MI:STC
550 Sender frequency limited
The email sending account frequency limit has been exceeded.
Stop sending emails, decrease the frequency, and resend the email later.
If the email sending account frequency exceeds the daily email sending quota, no email is allowed to be sent on that day.
The frequency limit is confidential data belonging to the recipient and is not publicly available.
Email sending domain name frequency limit exceeded 550 Domain frequency limited Email sending domain name frequency limit has been exceeded.
Stop sending emails, decrease the frequency, and resend the email later.
If the email sending domain name frequency exceeds the daily email sending quota, no email is allowed to be sent on that day.
The frequency limit is confidential data belonging to the recipient and is not publicly available.
Spam email 421 4.7.0 [xx] Message from ($IP) temporarily deferred
421 4.7.0 [xx] Message temporarily deferred
421 4.7.0 https://support.google.com/mail/answer/188131
450 Requested action not taken
451 DT:SPM
550 DT:SPM
550 SC-001
521 5.2.1 : AOL will not accept delivery of this message
550 5.7.1 Message rejected as spam by Content Filtering
550 Mail content denied
550 Suspected spam
554 Message not allowed
The email return code includes the following information:
as spam
is spam
spam denied
spam detected
spam filtering
suspected spam
contain the spam contents
The emails are suspected to be batch emails, are flagged as spam, or have been complained about by multiple users.
Advertisement and promotion emails may only be sent to subscribed recipients. Ask subscribed recipients to add the email sender address to their whitelist.
Ask recipients to add the email sender address for registration and notification emails to their whitelist. If the emails still cannot be received after adding the email sender address to their whitelist, ask the recipient to submit this issue to their service provider for support.
Email size exceeded 550 Message too large
552 Requested mail action aborted: exceeded mailsize limit
The email size is exceeded.
Decrease the email size.
Full mailbox 452 4.2.2 https://support.google.com/mail/?p=OverQuotaTemp
452 4.2.2 https://support.google.com/mail/answer/6558
552 5.2.2 https://support.google.com/mail/answer/6558
550 mailbox limit exceeded
552 mailbox is full
552 5.2.1 Quota exceeded or service disabled
The email return code includes the following information:
is over quota
exceed mailbox quota
The recipient’s mailbox is full.
Ask the recipient to clean up their mailbox.
Recipient temporarily unavailable 400 the mta server of unexpected closed
450 Service temporarily unavailable
451 $DOMAIN Resources temporarily unavailable
451 4.7.1 Sorry, the service is currently unavailable
451 Internal resource temporarily unavailable
451 Resources temporarily not available
451 Temporarily unable to process your email
The recipient system is temporarily unavailable.
Resend the email later.
If resending failed, ask the recipient to submit this issue to the email service provider for resolution.
Account greylisting 450 4.2.0 <$EMAIL>: Recipient address rejected
451 4.7.1 Greylisting in action
451 Grey bounce
The email return code includes the following information:
grey listed
The email sending account is greylisted by the recipient.
Ask the recipient to add the email sender address to the whitelist, and resend later.
If resending failed, ask the recipient to submit this issue to the email service provider for resolution.
Incorrect recipient address format 501 Bad address syntax
550 MI:IMF
The recipient address format is incorrect.
Confirm the correct email address with the recipient.
SPF validation failed 520 ip and spf record not match
550 MI:SPF
550 SPF check failed
SPF is set for the email sending domain name, but fails to validate.

Return codes of some email service providers

Email service provider Return code description
QQ Mailbox http://service.mail.qq.com/cgi-bin/help?id=20022
NetEase Mailbox http://help.163.com/09/1224/17/5RAJ4LMH00753VB8.html
263 Mailbox http://www.263.net/263/helpcenter/mailbounce/20150906/842.html
Sohu Mailbox http://mail.sohu.com/info/policy
Microsoft Mail (such as @hotmail.com, @live.com, @live.cn, and @outlook.com) https://mail.live.com/mail/troubleshooting.aspx#errors
Yahoo Mail https://help.yahoo.com/kb/postmaster/smtp-error-codes-sln23996.html?impressions=true
Aol Mail https://postmaster.aol.com/error-codes
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