I wonder if this is related to the problem Exchange Email Synchronisation Reliability - #5 by flypig ?
There does seem to be an issue with the way that the network stack operates within Sailfish, specifically maintaining an active connection when the underlying network transport changes from WIFI to mobile, or 2G/3G to 4G (when returning from a phone call for example) or any combination of change, even momentarily (e.g. a dropout on wifi switches to mobile, etc).
There appears to be a number of bugs, like the one above, where the root cause looks like it could be nothing to do with Exchange (as in the bug above) or phone calls, or mobile data in Android, etc but perhaps with the way the network stack itself works?
I often lose mobile connectivity in Android apps and I only have a single SIM XA2 on 4.1 - but I get this issue far less if I disable wifi …