One thing you may notice is that the new
mailman.ampr.org list host is
using 'greylisting' to filter out some spam; this means that the first
time you post to the list after not having done so for some days, your
posting will be delayed for some minutes.
Also, many of your receiving hosts are doing the same thing, so until they
learn the new list sending address, list mail will be delayed reaching
you too. See the Wikipedia article on 'greylisting' for a more complete
explanation of what's going on.
https://en.wikipedia.org/wiki/Greylisting
Our retry delay is set to 15 minutes, a common value.
- Brian