I think I should summarise on information required here.

1) Which OS are you using - uname -a
2) Which MTA are you using - i.e. sendmail/postfix
3) What does your nsswitch.conf look like - grep '^hosts' /etc/nsswitch.conf
4) What does your resolv.conf look like - cat /etc/resolv.conf (please obscure the IPs and domain name, etc.)

We cannot assist without accurate and thorough information on what your current setup is.

Cheers,
ZB