In my opinion, one of the great underappreciated Microsoft Knowledgebase articles is KB147093 which explains one of those mysteries you see with Exchange servers from time-to-time.
The symptom is this - a remote sender transmits a message to multiple recipients on your Exchange server, but one or more of the recipients is incorrect. This causes the mail transaction to fail and NO recipients get the message.
Although KB147093 refers to X400, in fact this is the behaviour that you'll see on an Exchange 5.5 Internet Mail Connector, and it works with other SMTP-based mail servers too.
The problem is this - when sending to multiple recipients at one remote domain, the software at the sender's end will make a single connection to the remote mail servers.. and it's an all-or-nothing proposition.
The problem is compounded if you suppress NDRs (nondelivery reports) to the internet, because a remote sender will never receive a bounce message to say that the mail transaction failed. In these circumstances, it can take some time to work out that there's a problem at all.. but in this case you need to carefully check the recipient list for invalid users and remove them.
Now, if you have NDRs enabled, the problem will probably be spotted much sooner. But these days a lot of organisations turn them off, especially if they are the targets of mass spamming or directory harvesting attacks. It's one of those cases where the current levels of spam have unexpected adverse impacts on infrastructure.