I've been thinking this for years, but never bothered to make this comment until now:
Wouldn't it be great if the Mail Queue Manager showed which messages on the queue were inbound / incoming, and which were outbound / outgoing? Just bolding the incoming messages would a good start. Or perhaps give three views - Show All, Show Inbound, Show Outbound.
One of the most obvious benefits of this would be diagnostic: if a message is inbound, then what's it doing on the queue, i.e. why wasn't it delivered immediately?! (usually because the POP account is over-quota). Hell, you could go a step further and list next to each message why it's on the queue (i.e. the most recent reason for not being delivered immediately), as opposed to having to click the 'Deliver Now' button alongside each message to find this out. Nothing impresses a hosting customer more than when I proactively contact them to let them know there's a problem with their mail.
There is one potential issue I can see here - when there's more than one recipient for a message, some of the recipients could be incoming/local, and some outgoing/remote.
Is it just me, or this a worthy suggestion for a new WHM feature? :-)