The below is a quick reference to common Mail Delivery Errors, often known as SMTP status or SMTP error codes for SMTP mail servers such as AA Mail Server, GMS Mail, Lotus Notes, Microsoft Exchange, PowerMTA, SendMail, WinMail, Zimbra or any other SMTP/ESMTP standards compliant e-mail MTA.
These codes are not a definitive golden rule but more of a guideline as each mail server and mail transfer agent (MTA) have their own classification that they report back to our Send system.
Important note: Sometimes it can seem as if the errors delivered do not correspond to the actual error occurring. For example, the report delivers an error "bounced: invalid email" when you have verified the email address is correct. In these situations it is important to note that Concep Send pulls through the errors directly from the email client or inbox vendor. Sometimes they will deliver an error that is not a perfect description of the error in order to simply convey the message that the email has not been delivered.
Example SMTP Mail Delivery Errors
| SMTP Status Code | Alternative Code Name | Message Meaning |
| 101 | 1.0.1 | Cannot open connection |
| 111 | 1.1.1 | Connection refused |
| 252 | 2.5.2 | Cannot verify the user - the server will accept the message and attempt to deliver it |
| 421 | 4.2.1 | Service not available - the sending email program should try again later or The SMTP service/server you use has a limit on the number of concurrent SMTP streams your server can use |
| 422 | 4.2.2 | The recipient's mailbox is over it's storage limit or The size of the message exceeds the recipient's size limits for incoming emails |
| 431 | 4.3.1 | The recipient's mail server is experiencing Disk Full condition |
| 432 | 4.3.2 | The recipient's Exchange Server incoming mail queue has been stopped |
| 441 | 4.4.1 | The recipient's server is not responding |
| 442 | 4.4.2 | The connection was dropped during transmission |
| 446 | 4.4.6 | The maximum hop count was exceeded for the message |
| 447 | 4.4.7 | Your outgoing message timed out |
| 449 | 4.4.9 | Routing error |
| 450 | 4.5.0 | Requested action not taken - The mailbox was unavailable at the remote end |
| 451 | 4.5.1 | Requested action aborted - Local error in processing or Requested action delayed - Local problem or <IP address> has recently sent spam |
| 452 | 4.5.2 | Requested action not taken - Insufficient storage |
| 465 | 4.6.5 | Code Page unavailable on the recipient server |
| 471 | 4.7.1 | This is a local error with the sending server and is often followed with 'Please try again later' |
| 500 | 5.0.0 | Syntax error command not recognized |
| 501 | 5.0.1 | Syntax error in parameters or arguments (e.g. email address) |
| 502 | 5.0.2 | Command not implemented |
| 503 | 5.0.3 | Bad sequence of commands or This mail server requires authentication |
| 504 | 5.0.4 | Command parameter not implemented |
| 510 | 5.1.0 | Bad email address |
| 511 | 5.1.1 | Bad email address |
| 512 | 5.1.2 | The host server for the recipient's domain name cannot be found (DNS error) |
| 513 | 5.1.3 | Address type is incorrect (most mail servers) or Relaying denied or Authentication required (a small percentage of mail servers) |
| 523 | 5.2.3 | The recipient's mailbox cannot receive messages this big |
| 530 | 5.3.0 | Authentication is required or Your server has been blocklisted by the recipient's server or The recipient's mailbox does not exist |
| 541 | 5.4.1 | Recipient address rejected - access denied |
| 550 | 5.5.0 | Requested actions not taken as the mailbox is unavailable |
| 551 | 5.5.1 | User not local or invalid address - relay denied |
| 552 | 5.5.2 | Requested mail actions aborted - exceeded storage allocation or Size of the incoming message exceeds the incoming size limit |
| 553 | 5.5.3 | Requested actions not taken - mailbox name invalid or You are attempting to send emails through a specific ISP's SMTP server without authentication and without being connected to the Internet through that ISP's service or You are sending from an Exchange Server configured to send via DNS and you do not have a public reverse DNS record pointing back to your exchange server |
| 554 | 5.5.4 | Transaction failed - most cases where the recipient server believes your email is spam or your IP address or ISP server has been blocklisted on one or more Internet blocklists or Yahoo - email address does not exist or is disabled or IBM Lotus Domino - either a Domino bug or a Disk Full error |
| 571 | 5.7.1 | Your server is not permitted |
If you wish for further details please take a look at this SMTP codes document.
That response occurs when the email server sending us the bounceback message does not provide a reason - meaning all it reports back to our servers is that the email is not deliverable, and will never be. This could be due to a spam filter or other corporate email filter setting.
In these cases, we recommend switching the contact to plain text format and resending the campaign to see if it can be delivered. Plain text, while not as visually appealing, is the simplest form of email and is almost always not a problem with spam and corporate email filters.
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