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[ Tried this over on mailop; no response, so now trying here. ]

I've noticed that one of my servers has been unable to establish port 25
connections to hosts such as for over a week...and I'm
entirely puzzled as to why, since it only sends a trickle of traffic
to a handful of users etc. (They're on a couple of
small, low-volume Mailman-managed lists, and have been for time periods
ranging from "over a year" to "most of a decade" without any issues.)
Some help figuring out why this is happening would be entirely welcome.