Paging postmaster at gmx.net/gmx.de et.al.

[rsk at gsp.org (Rich Kulawiec)]

[ 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 mx00.emig.gmx.net 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 @gmx.net/@gmx.de 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.

Thanks,
---rsk