SMTP Error Codes

When an SMTP server accepts or fails any emails, there are three main types of codes used; 200, 400 and 500 codes. The first number in this code specifies if the MTA (Mail Transfer Agents) either accepted or rejected the email. 

SMTP code 2XX

Successful Delivery -- The email has successfully been accepted for onward delivery.

SMTP code 4XX

Persistent Transient Failure -- If you receive an error code containing 4XX, this means there is a temporary error that is causing the mail not to be accepted for delivery.

SMTP code 5XX

Permanent Failure -- If you receive an error code containing 5XX, this means there is a permanent error and the message is permanently rejected.

SMTP Code 400

Error Code
  • 421
  • 432
  •  Domain Service not available, closing transmission channel
Domain could be temporarily down. It's a Microsoft Exchange Server's SMTP error code. You should contact it to get more information: generally, it's due to a connection problem.
  • 441
  • The recipient’s server is not responding
The recipient’s server could be offline or having technical issues.Repeated attempts to send e-mail messages to this mailbox can be successful once the server is online.
  • 450
  • Requested mail action not taken: mailbox unavailable. request refused
This may happen as a result of cleaning dead addresses from the server, the mailbox is corrupted, or the mailbox is on another server that is temporarily unavailable.Repeated attempts to send e-mail messages to this mailbox can be successful.
  • 451
  • Requested action aborted: local error in processing Request is unable to be processed, try again
  • There is a server error on the recipient’s side, which causes the command to be aborted.
It's normally a transient error due to a message overload, but it can refer also to a rejection due to a remote antispam filter. If it keeps repeating, ask your SMTP provider to check the situation
  • 452
  • Requested action not taken: insufficient system storage
  • There is not enough storage on the server.
The recipient’s server does not have enough storage, once space becomes available, the emails will be sent.
  • 453
  • The host on which the mailbox is resident is not accepting messages
This could be a result of an imminent shutdown, excessive load, or system maintenance on the recipient's end. 
  • 454
  • TLS not available due to temporary reason. Encryption required for requested authentication mechanism
Something related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes. This status code may also be used when the condition cannot be further described because of security policies in force.
  • 458

  • Unable to queue messages for node
Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.
  • 459

  • Node not allowed: reason
Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.


SMTP Code 500

Error Code    
  • 500    
  • Syntax error, command unrecognized
  • A syntax error: the server couldn't recognize the command.
It may be caused by a bad interaction between the server with your firewall or antivirus.
  • 501  
  • Syntax error in parameters or arguments
  • Another syntax error, not in the command but in its parameters or arguments.
In the majority of the times, it's due to an invalid email address, but it can also be associated with connection problems.
  • 502
  • Command not implemented
The command has not been activated yet on your own server.
  • 503
  • Bad sequence of commands
  • The server has encountered a bad sequence of commands, or it requires an authentication.
In case of "bad sequence", the server has pulled off its commands in a wrong order, usually because of a broken connection. If an authentication is needed, you should enter your username and password.
  • 504
  • User account has been disabled by the mail administrator

You will need to remove the mailbox authentication lock from the mailbox in the mailbox properties within the ME Admin MMC.
  • 510
  • Check the recipient address
  • Bad email address.
One of the addresses in your TO, CC or BBC line doesn't exist.
  • 512
  • Domain cannot be found. Unknown host.
  • A DNS error: the host server for the recipient's domain name cannot be found.
Check again all your recipients' addresses: there will likely be an error in a domain name (like mail@domain.coom instead of
  • 513 / 515
  • Destination mailbox address invalid
  • Address type is incorrect
Doublecheck your recipients' addresses and correct any mistake. If everything's ok and the error persists, then it's caused by a configuration issue (simply, the server needs an authentication).
  • 521
  • Domain does not accept mail
Check if the recipients' domain is active and has not been blacklisted. The recipient's Admin will have to troubleshoot further.
  • 522
  • Recipient has exceeded mailbox limit
The recipient's mailbox has reached their limit. They will need to up their limit or create space to start receiving email.
  • 523
  • Server limit exceeded. Message too large
  • The total size of your mailing exceeds the recipient server's limits

Re-send your message splitting the list into smaller subsets.
  • 530
  • Access Denied. Authentication required
  • Normally, an authentication problem. But sometimes it's about the recipient's server blacklisting yours, or an invalid email address.

Configure your settings providing a username+password authentication. If the error persists, check all your recipients' addresses and if you've been blacklisted.
  • 531
  • Mail system Full
The recipient's email server is not accepting any emails. Their Admin will need to create space of the server or troubleshoot further.
  • 533
  • Remote server has insufficient disk space to hold email
The recipient's email server does not have enough disk space. they need to create more space.
  • 534
  • Authentication mechanism is too weak. Message too big
The sender's email was too larger for the recipient's email server.
  • 538
  • Encryption required for requested authentication mechanism
The recipient's email server requires encryption.
  • 540
  • Recipient address rejected

The email was returned to you likely because the intended recipient's account has been suspended or deactivated. Their account won't be able to receive email until the suspension is lifted and the account reactivated.
  • 541
  • The recipient address rejected your message: normally, it's an error caused by an anti-spam filter.
Your message has been detected and labeled as spam. You must ask the recipient to whitelist you.
  • 542
  • Bad Connection
Check internet connection.
  • 543
  • Routing server failure. No available route
Check internet connection.
  • 550
  • Requested action not taken: mailbox unavailable
  • It usually defines a non-existent email address on the remote side.
The recipient email address doesn't exist. You should contact the recipient otherwise and get the right address.
  • 551
  • User not local or invalid address – Relay denied
You should contact your ISP and ask them to allow you as a certified sender.
  • 552
  • Requested mail action aborted: exceeded storage allocation
Try to send a lighter message: that usually happens when you dispatch emails with big attachments, so check them first.
  • 553
  • Requested action not taken: mailbox name not allowed
Check all the addresses in the TO, CC and BCC field. There should be an error or a misspelling somewhere.
  • 554
  • Transaction failed
The incoming server thinks that your email is spam, or your IP has been blacklisted. Check carefully if you ended up on some spam lists.