[Evolution] How does evo determine when to base64 encode?

["Johnson, Brett E (HP Cloud Linux R&D)" <brett hpe com>]

Hi all,

Here at work, I've recently had a msexchange account shoved down my throat, and
I've noticed that, using evolution-ews, evolution almost always base64-encodes
the body of my messages.  Yet, if I send the same message via SMTP, it is not
encoded.  Can anyone explain this?  Is there any way to turn it off?  I really
don't want to send out base64 encoded blobs to anyone, especially if I have no
control or warning over it.

-- 
Brett Johnson<brett hpe com>
"It is far more useful to be aware of a single shortcoming in ourselves than
 it is to be aware of a thousand in somebody else. For when the fault is our
 own, we are in a position to correct it."
  ~~The Dalai Lama