Can't Send Large Messages Using Sendmail CentOS 5

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http://serverfault.com – I have recently replaced a Fedora Core 3 server with one running CentOS 5 for the purposes of a mail relay in a DMZ. My problem is that when the server is sending messages to a particular remote organizaion, messages larger than 500KB don't send properly. They seem to get hung up somewhere through the transmission, time out, then after the predictable retries, expire in the queue. Sendmail describes the problem as: MDeferred: Connection timed out with [site]. Large (or larger) messages send successfully to every other remote system we have tested with. It is just this single organization (HowTos)