I need to have non-delivery reports (NDRs) sent for emails that are too large instead of moving them into the TOOLARGE folder

POPcon itself can't send out NDRs but Exchange can. To enable those you need to set a very high size limit in the receive connector (so that it never stops an email just there) and then set the real limit in the either the global options or the individual users' mailbox settings.

In Exchange 2010 or 2007 the default settings in the receive connector are only to allow emails of less than 10MB in. You need to change this setting to a larger number, like 100MB like this: Open the Exchange Management Console under Server Configuration, Hub Transport, Receive Connectors, Properties of the Default Receive Connector (repeat this with all other connectors if you are unsure if it's the default connector that is active for connections from the same server):

The Maximum message size is fixed to 10240KB (10MB) by default on the General tab of these properties Change that to 102400 (100MB) in order not to allow longer emails to come through. You can always limit individual user’s allowed maximum email sizes.

Next go to Organization configuration > Hub Transport > Global Settings:

Please open the Properties of the Transport Settings:

You can set your real limit here under "Maximum receive size".

After that Exchange will accept large emails again from POPcon so POPcon doesn't need to move them into the toolarge folder. If the email is smaller than the Maximum receive size in the Transport Settings it will go through normally. If it's larger than that it will get sent back the sender with a nice non-delivery message from your Exchange.