This is going to cost them...
Down for several hours (and counting) for numerous customers (and that's just the ones who've managed to fight off the angry mobs of users long enough to tweet about it). That 100% SLA is looking a bit sick right now.
One of Mimecast's selling points was that they have a resilient infrastructure, so that the loss of a datacentre would just require a bit of re-routing to maintain service. Explanations and compensation will be expected by all.
(And yeah, we just renewed our contract after three trouble-free years)