First, always make sure to provide the version of Unity you're working with and a little info on the back end setup (Ex55 or 2K? on or off box?)
Next, couple places to check...
1. Check the unaddressed messages distribution list members (by default this is only the Example Administrator which Unity creates however you should have added at least one "real" admin to this group during initial configuration). If for any reason Exchange bounced the outside caller message back at us when attempting to deliver the message from the Unity Messaging system account, it'd get forwarded to this list.
2. Check the application event log for errors when you leave an outside caller message for this account. In particular, UMR errors since this is the guy that's responsible for handing messages off to Exchange for outside callers.
3. Check the \Commserver\UnityMTA directory for any messages there... this is where the UMR picks up messages and if they can't be delivered they'll sit in here or move to the \failed directory under it depending on what version of Unity you're at.
4. If the message is still AWOL after that, then it's very likely sitting in the companion Exchange server's MTA queue and not being delivered from there. This often happens as the result of DC issues of one sort or another since the MTA uses server names to do the delivery/bind back and if it can't resolve the name for whatever reason it'll fail. The event log on the companion Exchange server (the one you selected during Unity's configuration setup) will likely show RPC errors if this is the case.
Subscriber to subscriber messages are send from the sender's home Exchange server whereas outside caller messages are left from the companion Exchange server. As such one could be having problems and the other not which would explain this (i.e. if they were in different subnets and the DC servicing the companion server's subnet hit the dirt). Again, some more details about your back end configuration would help rule out such things or not...
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.