New message alert emails not being sent

I'm using v1.10-rc.3 (907ec36) and only updated from 1.10-rc.2 recently, but since updating I am no longer getting new message email alerts, I am getting other emails from the system so emails are being sent ok. I have checked to see if new message alert is still enabled in admin->settings->Tickets->Alerts and Notices, and it is still enabled.

Any ideas would be appreciated.

Many thanks.

Gareth