It looks like evolution uses for plain ASCII text html formating combined with
text plain both base64 encoded.  IMHO it is enough to use ASCII with quoted
high bits characters for ISO as well as for UTF-8 (multi byte) characters as
long as there are not real binary data around (like pictures).

--_000_1530545649596010camelsusecom_
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64

[... base464 ...]

--_000_1530545649596010camelsusecom_
Content-Type: text/html; charset="utf-8"
Content-ID:
<7F8C6FCC4481C84DAE16CA8F0B3C348A@namprd18.prod.outlook.com<mailto:7F8C6FCC4481C84DAE16CA8F0B3C348A@namprd18.prod.outlook.com>>
Content-Transfer-Encoding: base64

[... base64 ...]

--_000_1530545649596010camelsusecom_--