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?