I'd like to re-open this request, since the suggested solution of using Primary & Backup servers doesn't seem to work in my case.
I have one IPv6 server with ~900 days retention & 10 connections max & auth, and then I have my ISP's IPv4 server with ~30-60 days retention & 4 connections max w/o auth. My ISP's server temporarily blocks connections when hundreds of requests are made for missing article parts. Setting the IPv6 server as a backup doesn't work, since I'm already using the maximum allowed connections (failure to connect). Reducing it 6 connections IPv6 + 4 connections IPv4 (w/ 4 connections IPv6 backup) doesn't work either. A single part gets download and then failure to connect, and I wouldn't want to do this anyways since it hurts my overall speed significantly.
A way to automatically disable a server based on post age seems to be the optimal solution for this issue, so no missing part failures, fall-backs, or server thrashing ever occurs.
Other enhancements which may be useful:
An option to automatically re-try all missing/corrupt parts on each primary server once in succession, until success or all fail.
An option to set a global connection limit for all servers with the same IP/Domain, instead of adding more connections + logic which prioritizes connections to identical backup servers.
An option to temporarily disable a server for an NZB collection after a user set threshold for missing/corrupt parts is exceeded.
