reverse lookup sort of broken

I have a reeealy bizaar problem. My class C address space has been delegated to me and my nameservers are responding to forward and reverse querries. Recently I noticed that e-mail was boucing at various sites because they could not resolve my in-addr.arpa address. The entries exist in my reverse zone file. All of the entries in the zone respond except for the .10 and .11 address (which happen to be both of my e-mail servers), I get responsed on all of the other 15 entries. If I perform a dig on the nameserver locally I get responses for these two records. I also get responses for these two records if I query the nameservers from anywhere in my internal network. The only time there is a problem is from outside the firewall.

When I query the nameservers from the outside I get "server can't find x.x.x.x.in-addr.arpa: SERVFAIL". I get responses for any other host in the reverse zone (except) for these two.

Has anybody seen this before and have an idea on how to fix it??