I'd just like to wade in with a 'I have the exact same problem', we are Ireland. I have an almost identical setup at this site as denmark.
Enterprise Plan 2, 80+ Macs [Snow Leopard to Mavericks], Office 2011 14.3.8/9.
Getting the same -18500 error.
This started occurring Wednesday for us, 4th Dec.
Occurs when sending to external or internal sources, not sure why that would differ with Exchange online.
Does not occur when using OWA.
Regards, Neil.