The issue could be with the DNS cache they are using to resolve their DNS. Right now, I can resolve the A and MX records for honest-e.com using ns1.hannonweb2.net and ns2.hannonweb2.net (the authorative servers for that domain).
When your clients report this problem, you could try doing a DNS lookup yourself:
The output should be something like:
dig @ns1.hannonweb2.net. honest-e.com a
dig @ns2.hannonweb2.net. honest-e.com a
If the "answer" section is missing, or if the query hangs for ages, then the problem is most probably with the ISP's (hannonweb2's) DNS servers, and you should avail upon them to resolve the issue.
[root@vaio dns]# dig @ns1.hannonweb2.net. honest-e.com a
; <<>> DiG 9.2.1 <<>> @ns1.hannonweb2.net. honest-e.com a
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7762
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0
;; QUESTION SECTION:
;honest-e.com. IN A
;; ANSWER SECTION:
honest-e.com. 14400 IN A 126.96.36.199
;; AUTHORITY SECTION:
honest-e.com. 14400 IN NS ns2.hannonweb2.net.
honest-e.com. 14400 IN NS ns1.hannonweb2.net.
;; Query time: 104 msec
;; SERVER: 188.8.131.52#53(ns1.hannonweb2.net.)
;; WHEN: Sun Jul 6 17:04:27 2003
;; MSG SIZE rcvd: 96
If the query returns the answer section, then the problem is most likely with your client's DNS cache. If they can run the dig query and get the same results as you, then this is almost certainly the case. In this instance, you should either avail upon the DNS cache administrator to fix their DNS cache, or use a different DNS cache.