We ran into this in a few locations and I usually just use it as an opportunity to upgrade to the latest firmware.

I have not been available for half a day once when it happened, and the issue resolved itself in that instance.

It's very strange, and I have not experienced it on V6.

It also very rare to happen for me, maybe 3x across 15 SV9100's over 3 years. So I haven't invested much time in troubleshooting or reporting to NTAC.

Sorry that it's not much help, but I am more than happy to share my InMail config on a site that it has never happened to.