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#5.7.1 smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required

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  • #5.7.1 smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required


    I am having trouble with other servers rejecting emails. The recipients are large enterprise companies ( being one of them) so I assume they have strong mail filtering requirements. My SPF record has been in place for a long time but when I checked it with MXToolbox they said it was wrong so I recreated my SPF record and it checks out ok now. I do not have a DKIM or DMARC record setup, the first time I ever heard of it was when I started researching issues why I might be getting blocked.

    The NDR report says that checked my SPF record and it passed.

    The error message I am getting is < #5.7.1 smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required> and the generating server is the far end, not my own. Other discussions about this error point to the far end that they are blocking anonymous requests but I doubt that it is the other organizations that I am emailing.

  • #2
    Do you have use of a SMTP Relay server? If you do add a temporary SEND Connector for this domain and send via "Smart Host" this may help to determine if it's your end or theirs.

    What mail server and operating system versions are using?