"Out of Office" replies go to internal users but not to external internet senders

This is not a POPcon issue — it is an Exchange configuration setting. By default, Exchange does not send out-of-office replies to external senders.

Enable external out-of-office replies

Exchange 2003

1. Open Exchange System Manager.
2. Go to Global Settings > Internet Message Formats.
3. Double-click the default entry.
4. On the Advanced tab, check "Allow out of office responses".

Exchange 2007 / 2010 / 2013

Use the Exchange Management Shell: Set-RemoteDomain Default -AllowedOOFType All