SMTP Error Examples

451 Internal resource temporarily unavailable

This indicates a problem on the receiving server – there may be congestion or overloading which is preventing it from receiving the message at this time. Sometimes generating these messages is an attempt to avoid Spam, as spammers will often skip servers that generate these messages. If your message fails, the recipient may need to be contacted another way so their IT department can resolve the issue.

550 User unknown.

This indicates an error with the email address – either the address is incorrect or it may not be entered in the recipients server properly.

511 http://www.spamhaus.org/SBL/sbl.lasso?query=***** (where ***** is some alphanumeric string)

Indicates that the sending server has been blacklisted for sending Spam by SpamHaus (a reputable authority on the matter).

534 Message header size, or recipient list, exceeds policy limit.

Indicates the message header size exceeds the line length or size limit. This may be an error with the mail client sending the message, the server, or simply that the sender has sent the message to too many people. Bcc’ing recipients is better for their privacy / security and will resolve this issue.

417 DNS lookup failure: example.org. Try again later.

Indicates a problem with resolving the domain. Either the DNS servers may not be working, may be overloaded, or may be incorrectly configured.

517-Domain does not exist: ***** (where ***** is a domain name)

This indicates that the domain specified does not seem to exist.

517 Invalid domain, see ftp://ftp.isi.edu/in-notes/rfc1035.txt" data-mce-href="url:ftp://ftp.isi.edu/in-notes/rfc1035.txt">URL:ftp://ftp.isi.edu/in-notes/rfc1035.txt

The DNS for the recipients domain is not correctly configured. It MAY seem to work for them, but is configured ambiguously. They may not be aware of the issue. We can help explain further if required.

523 Message length exceeds administrative limit.

The message size exceeds the allowed limit. If you are sending multiple attachments, consider breaking into two or more separate emails, or using a file transfer service.