We have two internal DNS servers. One is located on a linux server box and the other is the router's DNS management. We set the linux box as primary DNS via DHCP and the router as secondary. We have a few Mac clients that are accessing our internal mail server (hostnamed "mail" internally).

When using IMAP or SMTP against the mail server internally, the mac boxes will sometimes fail to locate the server. If I use NSLOOKUP I can see that "mail" is pointed to the correct IP address and is being resolved via the correct DNS server, but if I ping "mail" it fails.

~ (bash)$ nslookup mail
Server:     254.254.254.206
Address:    254.254.254.206#53

Name:   mail.example.com
Address: 254.254.254.205

Note: I replaced our actual internal IP address with 254.254.254.*

If I wait a few minutes (3-5 minutes), somehow it resolves itself and sends successfully. This happens multiple times a day. The /etc/hosts file on the mac boxes is the default config.

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
255.255.255.255 broadcasthost
::1             localhost 
fe80::1%lo0 localhost

Is there something about Mac clients I should know to prevent this failed DNS resolution?

Client boxes are: OSX 10.7.4, 8GB RAM, i5 MacBooks Server is: Ubuntu 12.04 Server