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CentOS | Sendmail 8.14

Hi All,

I couldn't find this in the batbook. If SendMail attempts to deliver an email to a primary MX record and gets a 400-class error back, how quickly does it try the secondary mx (assuming the secondary record is present)?

1 Answer 1

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I found the following in the batbook (page 329):

The sendmail program then tries to deliver the message to each host in the list of MX hosts, one at a time, until one of them succeeds or until they all fail. The value of an MX record contains a cost value (also called preference) and the hostname to which to connect. All MX hosts at a given cost (preference) are tried before any at a higher cost (lower preference) are tried (that is, all the 5’s are tried, for example, before any 6’s). Beginning withV8.8 sendmail, if a host in the list returns a 5xy SMTP code (permanent failure), the effect is to cause subsequent MX hosts to be ignored. (Connect failures are the exception, in that they continue to the next MX host as usual.) Most temporary errors cause sendmail to try the next MX record. If sendmail exhausts the MX list with neither success nor a permanent error, the temporary error will cause the message to be queued for a later attempt.

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