In at least one envoronment, repeatedly, e.g. in this one: 

when attempting to subscribe to

https://lists.gnu.org/mailman/listinfo/bug-wget

Secret Agent Spoofer was the suspect cause that it wasn't possible for me to do so. Because of the X-Forwarded-For likely...
As soon as I removed SAS (I'm not advanced), I subscribed to that page just fine.

I have network traces and screencast that show/indicate that to be the case, and I decided to show them to you so that if that prooved to be the case, you can have the time to fix the eventual bug, and then only I would publish the trace/cast pair.

( I do those with my (primitive) program:
https://github.com/miroR/uncenz )

Encrypt email to me with my key:
0x4FBAF0AE
so I can post encrypted trace/cast pair to a new dir under, that only you can decrypt:
http://www.croatiafidelis.hr/foss/cap/
or tell me you don't mind, and that I can publish my (primitive, uncertain, beginner) research, without you looking at it first.

Thank you!
Miroslav Rovis
www.CroatiaFidelis.hr