SMTP Error Code 512

Error code
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SMTP Error : 512 – The host server for the recipient’s domain name cannot be found (DNS error)

Solution:
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This SMTP reply code is received when one of the servers on the way to the destination is unable to resolve the domain name of a recipient email address. If one of the servers on the way to the destination, including your server or your ISP, has a DNS problem or possibly correctly, does not like one of the email addresses in the messages TO, CC, and BCC fields.

The first check you should perform to resolve a 5.1.2 reply code is to check all the recipient email addresses for incorrect domain names (misspelt domain names or maybe totally non-existent domain names). This error code 512 is very specifically an error with the domain name of one of the recipient email addresses. You can check the recipients or use the WHOIS tool of The Ultimate Troubleshooter. If all the recipient email addresses check out as regards the domain part of the email addresses, then one of the servers on the way to the recipients has DNS problems. This will be one of the first 2 servers in the chain, your own mail server (or your network) or your ISP’s mail server.

Examples of typical SMTP error 512 messages :
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“5.1.2 – Bad destination host ‘DNS Hard Error looking up domain”

“SMTP Error 550 5.1.2 Host unknown – host cannot be found”

“5.1.2 The message could not be delivered because the recipient’s destination email system is unknown or invalid”.