It looks like a bug.
The only difference is that I was running on an SMP kernel (only 4GM of mem showing in free).
I switched to the PAE version of the same kernel in order to make the full system memory available (24GB).
I just booted back to the SMP version and every thing is back up.
I'll open a bug report on 5.0.9 and PAE for 24GB systems unless someone can point me in the direction of a config that I can update to prevent the problem.
I did not need to add a bug report its;
Bug 33485 – Limit max heap size on systems using 32-bit and PAE and greater than 4GB ram
I'll try dropping mailboxd_java_heap_memory_percent from 30 -> 10% and see what happens in the AM.
My users don't like to be without their e-mail.
uxbod. I see your reply below which jives with the bug report. Thanks