I have many computers at computer lab. Those computers have Windows Vista and
Office 2007. Infrastructure network contains Windows 2003 Active Directory,
and ISA Server 2006. All computers have ISA Firewall client installed. For
this reason, neither computers have been configured with a IP as gateway on
their NIC.

Outlook 2007 is configured to send and received e-mails using POP3 and SMTP.
When I configure address for POP3 server (pop3.domainname.com) or SMTP
(smtp.domainname.com), Outlook displays an error, show me that the e-mail
server could not be find it. However, if I use the numerical address
(67.170.11.23) instead DNS name, Outlook can send and receive e-mails.

I view that if I configure the computer using a IP as gateway (for example,
Internal IP of ISA Server), Outlook can resolve POP3 or SMTP names.

Could you explain me why occurs that?

Thank you!