Identical setups but Sendmail works only once

[bidevi]

I've got Fedora Core 6 setup identically on two machines but Sendmail works on only one machine.

I use a perl cgi mail form script. Sendmail has been modified according to guidelines I found here:

Smarthost for sendmail and/or exim | Linux Operating system; Configuration, How to and news focused on (Fedora, Debian, Ubuntu, Centos)

so I can use my ISP's smtp server thanks to smarthost.

On my first machine it works flawlessly. Someone sends the message from my web page, gets a confirmation message on his screen, gets sent a confirmation message to his email address and I receive the main message.

For my second machine the sender sees the same confirmation on his screen but no messages are ever sent.

I've set up the second machine identically to the first one. Eventually, I even copied the /etc/mail directory from PC1 to PC2. No success!

Obviously, I did something right on my first machine but I don't know what so I cannot repeat it for the second machine. Who can help?

[bidevi]

As soon as I switched off PC1, sendmail worked on PC2.