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We're using sendmail as an MTA to send confirmation emails to customers that place orders on another server running IIS. Occasionally we see lines like this in the logs:

SYSERR(root): collect: I/O error on connection from <server redacted>, from=<mail address redacted>

They only happen occasionally so it's hard to trace what happened. The time/day doesn't seem to be a factor as they happen at different times during the day. Neither server experiences high load; both of them are basically over-spec'd servers.

Some posts have mentioned it might be a TCP/IP related issue so I've done a tcpdump between these servers and found nothing suspicious (no runts, dropped packets, etc).

Can someone point me in the right direction of what to check next?

  • Does it affect ALL emails/connections from the remote server? – AnFi Nov 6 '14 at 6:32
  • Andrzej, no only sporadic mails. There doesn't seem to be a pattern. – Tensigh Nov 6 '14 at 7:41

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