Gmail API - plaintext word wrapping

Are you running the content through a quoted printable encoder and sending the encoded content value along with the header or expecting the API to encode it for you?

Per wikipedia it seems like if you add soft line breaks with = less than 76 characters apart as the last character on arbitrary lines, they should get decoded out of the result restoring your original text.

UPDATE

Try sending with this content whose message has been quoted-printable encoded (base64 it):

From: Example Account <[email protected]>
To: <[email protected]>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Subject: This is a test!
Date: Tue, 18 Oct 2016 10:46:57 -GMT-07:00

Here is a long test message that will probably cause some words to wrap in =
strange places.