I am trying to switch email servers from one machine to another. One has a older version of postfix. I got the firewall to forward all port 110 and 25 traffic to the new postfix email server and got everyone's outlook settings to pull mail from the new email server however, if the old email server is physically disconnected from the network, it takes at least a minute or so after people click on the "send/receive" button in outlook to get their emails. When the old email server is on the network it takes a second, even if the postfix services are stopped. What could be causing this? I looked through all the relevant *.conf files I could find, resolv.conf had the old email server as a nameserver but I changed it and it still has the same problem. I'm thinking it is something in the qpopper settings, but I dont know where the configuration files are.

This won't solve your problem, but, to be exact,
postfix is a SMTP server (port tcp/25), and is not used
to read your mailbox (incoming mail).

You need a POP or IMAP server (dovecot, imapd, qpopper, ...).

Now, you should investigate if your long timeout is related to
postfix (sending mail) or qpopper (checking mailbox).

Hope this helps...
Emmanuelito

the long timeout is related to qpopper,