[bitfolk] Secondary DNS: How long to keep expired zones?

Hi, Assume that: 1) You're a user of BitFolk's secondary DNS service. 2) One or more of your zones have been unreachable from our secondary servers for so long that they have expired. How long would you expect BitFolk to continue serving SERVFAIL for your zone(s) before removing them from the configuration (and thereafter serving NOERROR/REFUSED as for any name they're not authoritative for)? If any of you use other similar services where you provide the primary DNS server, what do they do in that circumstance? Is there a difference in behaviour in typical Internet protocols that encounter SERVFAIL vs NOERROR? e.g. would a mail server consider SERVFAIL to be a temporary failure but NOERROR to be a permanent one? Cheers, Andy -- http://bitfolk.com/ -- No-nonsense VPS hosting