Why can't I send email from one server but I can from another?

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http://serverfault.com – I'm trying to send email from PHP using mail(). I have complete access to both servers. No unusual iptables rules on either (nothing blocking a default install of sendmail) I installed sendmail on the one server a couple of years ago (Ubuntu 10.10) and that sends mail from PHP just fine. The new server (Ubuntu 10.04, set up this week) will not send mail - or it's trying, but every recipient is refusing to receive. I've checked out the sendmail logs on the new server and it's showing all of the MX servers of the recipient domains are Connection refused... There's no differences in the set up (HowTos)