When I do a dig for MX records on a non-auth server for a domain hosted
on a set of particular public DNS servers any out-of-zone data which
appears on the answer section isn't resolved in the additional section.
However when I repeat the dig a few seconds later the missing
out-of-zone data is resolved in the additional section. By example:

dig @ns.dns.co.uk foo.co.uk mx

; <<>> DiG 9.2.3 <<>> @ns.dns.co.uk foo.co.uk mx
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39775
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 4, ADDITIONAL: 5

;; QUESTION SECTION:
;foo.co.uk. IN MX

;; ANSWER SECTION:
foo.co.uk. 85592 IN MX 10 mail.foo.co.uk.
foo.co.uk. 85592 IN MX 100 backupmx-pri.bar.net.
foo.co.uk. 85592 IN MX 100 backupmx-sec.bar.net.

;; AUTHORITY SECTION:
foo.co.uk. 85592 IN NS auth1.dns.bar.net.
foo.co.uk. 85592 IN NS auth2.dns.bar.net.
foo.co.uk. 85592 IN NS auth3.dns.bar.net.
foo.co.uk. 85592 IN NS auth4.dns.bar.net.

;; ADDITIONAL SECTION:
mail.foo.co.uk. 85592 IN A x.y.14.226
auth1.dns.bar.net. 168464 IN A x.y.255.2
auth2.dns.bar.net. 168464 IN A x.y.255.6
auth3.dns.bar.net. 168464 IN A x.y.64.110
auth4.dns.bar.net. 168464 IN A x.y.255.94

;; Query time: 8 msec
;; SERVER: 132.185.132.21#53(ns.dns.co.uk)
;; WHEN: Wed Dec 8 11:11:08 2004
;; MSG SIZE rcvd: 283

hostmaster@dns-master /var/hostmaster/Customers/exec01/email $ dig
@ns.dns.co.uk foo.co.uk mx

; <<>> DiG 9.2.3 <<>> @ns.dns.co.uk foo.co.uk mx
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63489
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 4, ADDITIONAL: 7

;; QUESTION SECTION:
;foo.co.uk. IN MX

;; ANSWER SECTION:
foo.co.uk. 85587 IN MX 10 mail.foo.co.uk.
foo.co.uk. 85587 IN MX 100 backupmx-pri.bar.net.
foo.co.uk. 85587 IN MX 100 backupmx-sec.bar.net.

;; AUTHORITY SECTION:
foo.co.uk. 85587 IN NS auth1.dns.bar.net.
foo.co.uk. 85587 IN NS auth2.dns.bar.net.
foo.co.uk. 85587 IN NS auth3.dns.bar.net.
foo.co.uk. 85587 IN NS auth4.dns.bar.net.

;; ADDITIONAL SECTION:
mail.foo.co.uk. 85587 IN A x.y.14.226
backupmx-pri.bar.net. 295 IN A x.y.255.205
backupmx-sec.bar.net. 295 IN A x.y.255.206
auth1.dns.bar.net. 168459 IN A x.y.255.2
auth2.dns.bar.net. 168459 IN A x.y.255.6
auth3.dns.bar.net. 168459 IN A x.y.64.110
auth4.dns.bar.net. 168459 IN A x.y.255.94

;; Query time: 6 msec
;; SERVER: 132.185.132.21#53(ns.dns.co.uk)
;; WHEN: Wed Dec 8 11:11:12 2004
;; MSG SIZE rcvd: 315

This problem is specific to one ISP's nameservers and is possibly the
cause of some problems I'm having mailing companies whose domains they
host.

Anyone come across this before?