My email sometimes comes through garbled - is it encoded?

[DanC]

Now and then an email I'll get comes through as a long list of meaningless
letters and numbers. Is it encoded? How can I turn this off?

At the top of such emails, it says:

This is a multipart message in MIME format.
--=_alternative 0045564FC1257137_=
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: base64

Please help!