This should only be seen when the mime type/transfer encoding is either ignored (by receiver) or misplaced (by sender).
So it depends on the mailers involved..., if you post process the message yourself (raw format, you need to take into account the Transfer-encoding).
You can check in the raw message text, in the header there should be a mime header like.
Content-Type: text/html;
charset="ISO-8859-1"
Content-Transfer-Encoding: quoted-printable
or:
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
There might also be Content-Type: multipart/mixed somewhere with separators if the message has attachments. (or both a text and a html part)
When content then has a line ending in '=' the output will continue the the remainder of next input lines, =20 is ' ' (space_, =3d is '=', etc.)