Hello,
I have experienced a similar issue with the HP M525 which may not relate.
In our situation all scanning went down simultaneously for this particular model until we went out and reconnected them to the network. As soon as we applied a new network setting to the printer it came back online and restored the email settings we had set for the device. Since we use HP's Digitial Sending Service (DSS) to manage our network scanners we learned that there is actually a bug between the printer and DSS that was responsible for our outages.
The solution for us was to force the new printers to retrieve email settings directly from DSS and not from the printer itself. Anything we configured on the printer appeared to override itself and resort back to the defaults from time to time. This solution has been solid for us for many weeks now.
Let me know if you are using DSS or not and I will provide more details on where to make the change if necessary.