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Some recipients do not receive emails from my Postfix 2.11.0 server. Their servers reject my SASL connections with errors like 535 5.7.8 Error: authentication failed: authentication failure and 535 5.7.0 authentication failed. Full log:

postfix/smtpd[7847]: connect from ip6-localhost[127.0.0.1]
postfix/smtpd[7847]: C5FCBC073D: client=ip6-localhost[127.0.0.1]
postfix/cleanup[7850]: C5FCBC073D: message-id=<20151125093137.2433.71734@django>
postfix/qmgr[1897]: C5FCBC073D: from=<[email protected]>, size=5985, nrcpt=1 (queue active)
postfix/smtpd[7847]: disconnect from ip6-localhost[127.0.0.1]
postfix/smtp[7851]: C5FCBC073D: SASL authentication failed; server server44.firstfind.nl[93.94.226.70] said: 535 5.7.8 Error: authentication failed: authentication failure
postfix/smtp[7851]: C5FCBC073D: SASL authentication failed; server mx1.firstfind.nl[85.158.200.78] said: 535 5.7.0 authentication failed
postfix/smtp[7851]: C5FCBC073D: to=<[email protected]>, relay=mx2.firstfind.nl[85.158.202.246]:25, delay=4.7, delays=0.02/0.02/4.7/0, dsn=4.7.0, status=deferred (SASL authentication failed; server mx2.firstfind.nl[85.158.202.246] said: 535 5.7.0 authentication failed)

Gmail users receive these emails as expected, though. What's going on here? Is this a configuration issue on my end, or a problem with the receiving servers?

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Why are you using authentication? Most email does not use authentication.

  • The credentials and method you are using must be enabled on the receiving server. Your server must provided credentials that have been configured in the receiving server's authentication stores.
  • Authentication should only be done if the server announces that it accepts authentication.
  • Authentication generally requires a TLS connection so that credentials can not be sniffed. (The certificate may not be part of the public PKI infrastucture.)
  • Authentication may be enabled only on the submission port.
  • Authentication is not required to relay mail to the MX for the domain you are sending to.
  • Authentication is generally required to send mail that will be relayed to a domain that the mail server is not an MX (Mail Exchange) or MDA (Mail Delievery Agent) for. This authentication may be IP based allowing servers within the local network to relay mail without authenticating.
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