[Evolution] html messages converted to plain text?

[Mike Wilson <mwilson ketchumtrading com>]

I'm having an issue sending HTML messages. They look fine in the
editor, yet the recipients always receive them as plain text. It
doesn't matter if it's a new message or if I'm forwarding an existing
HTML message. They all get converted to plain text. I'm not sure if
this is a bug or just a configuration issue on my part.

I've set the composer settings to default to html. I've tried switching
the settings with the drop down menu in the composer - that does change
the composer, but what gets sent is always plain text.

Version info:

Ubuntu 16.04
Evolution 3.18.5.2
evolution-ews 3.18.5-1
Server is MS Exchange 2007